Freshly into his teens, he moved to the academy of renowned coach Riccardo Piatti, who has helped some of the greatest talents in recent memory, including Novak Djokovic, Maria Sharapova and Richard Gasquet. Viktor Galovic, who climbed as high as World No. 173, spent a year-and-a-half training there with a different coach. The Croatian recalls meeting Sinner when the Italian was 15.

“We were playing practice matches. I was No. 170, 180 at that moment, and he was almost beating me at 15, 16 years old, so he was already a prodigy,” Galovic said. “He was not winning so much, but in practice, you could see already that he was going to be something.

“He was already an amazing, amazing player, hitting amazing, and the timing on the ball was something incredible. He was lacking a little bit of a serve, but it was something that he would improve for sure, so it was just a matter of time.” 

Sinner first entered the PIF ATP Rankings on 12 February 2018 after winning a match in Sharm El Sheikh, Egypt against World No. 756 Aryan Goveas. Aged 16, the Italian defeated his Indian opponent 6-4, 6-4 to earn his first PIF ATP Rankings point.

“I remember speaking to my coach, saying that, ‘Oh, you know, I've got a good draw because I play a guy with no ranking’,” Goveas said. “I had never seen him before, so I checked a little bit of his junior [results], didn't show much… I got onto the court and I clearly remember playing a great match.

“But even at that age, he was so composed mentally, and he was so relaxed. It didn't look like that was the first [professional] match he'd played.”

The following February, still outside the world’s Top 500, Sinner competed in his first ATP Challenger Tour event of 2019 in Bergamo. In the third round, he faced a familiar opponent in Galovic, who was ranked more than 300 spots higher at the time.

“I was a break up in the third set, so I was confident that I was going to win the match. But during a changeover — we were friends at the time, still now — he smiled at me, so we smiled at each other. And then I lost the match,” Galovic recalled. “I remember that I was thinking, ‘Come on, this guy smiled after he was one break down in the third set! He is so relaxed, he is so confident that he is going to win the match.’ 

“I think this is the big key of Jannik. He's really confident that he really can beat everybody. And he was like this already when he was 16, 17.” 

That smile was an early sign that Sinner is not afraid of facing adversity. The teen won his first Challenger title that week, becoming the youngest Italian to triumph at that level in history.

Soon enough Sinner would be stepping on court with the best players in the world. He made his ATP Tour debut that April in Budapest, first ATP Masters 1000 appearance in Rome and even spent time on the practice court with Roger Federer during the grass-court season in Halle.

Sinner's rise happened quickly. He qualified for his first major main draw at the 2019 US Open, where he tested three-time Slam champion Stan Wawrinka in the first round.

That October, he competed in an ATP 250 event in Antwerp, where he advanced to his first ATP Tour semi-final. One of the 29 players who reached World No. 1 in the PIF ATP Rankings, Yevgeny Kafelnikov, was impressed with what he saw that week.

“[Mark] my words! This kid in three years world number 1,” Kafelnikov posted on X, then known as Twitter.

One year after attending the prestigious Next Gen ATP Finals presented by PIF as a fan, Sinner won the 2019 event. The next day he travelled to Ortisei, Italy, to play a Challenger rather than take holiday. He lifted the trophy.

By April 2021 — less than a year later in tennis time due to the Covid-19 pandemic — he cracked the world’s Top 20. It was intelligible that the Italian would at a minimum be a major piece of the sport’s future.

But Sinner never viewed himself through that lens. He did not believe he had accomplished much and only wanted to focus on the long road to maximising his potential.

“I try to improve every day, which is my main goal, and the results will come,” Sinner said when he cracked the Top 20. “Being a champion is a long, long road. It’s still a long way away… [I’ve made] a good start being 19 years old and playing at the highest level, but I don’t think about being a champion at the moment.”

Later that year Sinner served as an alternate at the Nitto ATP Finals, where he was slotted into the field due to a Matteo Berrettini injury. The home favourite pushed then-World No. 2 Daniil Medvedev to a final-set tie-break, giving fans a taste of what he was capable of. He was not scared of the moment.

The future of the sport was becoming the present, too.

That is why a social media post caught so many in the sport by surprise in February 2022. Sinner announced he had parted ways with his long-time team, led by Piatti. He would hire countryman Simone Vagnozzi and later Darren Cahill, the former coach of Lleyton Hewitt and Andre Agassi among others, to oversee his progress.

It was a bold decision for a 20-year-old. His results were not lacking — he had just made the quarter-finals of the Australian Open. There was no guarantee the change would help in the long run, but the Italian did it anyway.

"Riccardo Piatti at first was a great coach for him to get to the first stage,” Kafelnikov said. “And then obviously, when Darren Cahill took over, he just gave him something special on a better level, which he was missing."

Since joining Sinner’s team, Cahill has deflected all praise to Vagnozzi. Last year he said: “Ninety-nine per cent of the credit for the coaching goes to Simone. He's done an incredible job with Jannik.

“I'm more overseeing everything. Certainly we discuss tactics and the technical [side] and everything, but Simone is the voice. He's the guy driving a lot of what you see in the improvements of Jannik. He's doing an incredible job.”

The change would prove a critical moment. But the big wins took some time and there was still adversity to overcome. 

In that year’s Wimbledon quarter-finals, Sinner earned a two-sets lead against Djokovic, but won just seven games in the final three sets.

Then came the 2022 US Open quarter-finals. Sinner faced the one player, 19-year-old Carlos Alcaraz, rising more rapidly than he was. The Spaniard was already a human highlights reel, holding crowds at his fingertips with his electric shotmaking and ability to turn the ordinary to extraordinary in a split second.

Alcaraz won their marathon match in five hours and 15 minutes, with the final ball being struck at a tournament-record 2:50 a.m. The teen went on to win the title and by doing so became the youngest No. 1 in the history of the PIF ATP Rankings.

But with all the focus on Alcaraz, it was easy to miss that Sinner controlled a good portion of the match and held a match point when he served for a place in the semi-finals in the fourth set. It took a superhuman effort for the Spaniard to win that night.

“It was a good match for sure,” Sinner said. “But a very, very tough one.”

It was another dose of adversity for the rising Italian. 

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Kafelnikov, who has followed Sinner’s progress closely, said: “Absolutely the failure, which he had in those matches, I'm sure had some positive effects on his development. That's for sure. He started working even harder just to avoid [suffering] again those kinds of failures.”

Much of the attention goes to Sinner’s booming groundstrokes, but his indefatigable competitive spirit is one of his greatest attributes. Disappointment doesn’t keep him down, but motivates him to find a new level.

That extends beyond the court, too, even to the card games he often plays with his team. Last year Vagnozzi spoke to the ATP about his charge’s will to succeed.

“Jannik wants to win everything,” Vagnozzi said. “Just yesterday we lost 20 bucks each, Darren and I, because he hit the ball can with his serve on the first try. If we say to him, ‘You get 20 Euros if you do it the first time and 15 Euros if you do it on the second’, [he always] does it on the first!”

So when Sinner dropped to No. 17 in the world last year, it was never going to take him long to respond. Ever since, he has been a red-headed rocket ship blasting into the tennis stratosphere.

In the past year, Sinner has won his first two ATP Masters 1000 titles (Toronto and Miami), first major trophy at the Australian Open and three additional ATP 500 crowns. He also thrilled his home fans with a memorable run to the championship match at the Nitto ATP Finals, where only Djokovic was able to stop him after losing to the home favourite in group play.

Not only has Sinner been winning titles, but he has been beating the very best players in the sport. Backed by a retooled and improved serve, he has tallied a 15-4 record against Top 10 opponents. Nine of those victories have come against current or former World No. 1s: Djokovic (3), Alcaraz (1) and Medvedev (5). 

“I think his game has developed tremendously. You see in the results, you see in the level, he started to believe that he belongs to the top,” Kafelnikov said. “Where before, he was breaking Top 10 and maybe concerned about other players. But right now, he's on his own agenda. He's playing on his own terms, and that's what I think makes him special compared to what he was even last year. 

“He is just a different player, has just a different attitude and a different belief. I think that's what made him so good. And that's why we see where he is positioned.”

After advancing to the championship match in Turin, Sinner beat Djokovic twice in a day — in singles and doubles — while leading Italy to the Davis Cup trophy. The game was always there, and now he seemingly knew that he could beat anyone in the world on any stage.

Sinner began 2024 as World No. 4. At the Australian Open, his first tournament of the year, that was clearly underselling his level. He had a new swagger to him.

Sinner reached the Melbourne semi-finals without dropping a set, and then eased by 24-time major champion Djokovic in a four-set match that was never particularly in the balance.

“He was always very calm, very composed on the court, but I think he struggled maybe to win the big matches in the big moments,” Djokovic said. “But now it's coming together for him.”

In the final, an always witty former World No. 1 Medvedev played ultra aggressively to win the first two sets against Sinner. But while the Italian did not find a way early in his career in the biggest moments, he never panicked. Adversity? No problem. The 22-year-old allowed his game to do the talking, and that it did.

With the trophy in hand after a thrilling comeback, it became apparent it would be a matter of when, not if, Sinner would become World No. 1.

One year ago, Sinner suffered a disappointing second-round defeat at Roland Garros to Daniel Altmaier, an exit he reflected on last week in Paris.

"I think I've learned a lot from the losses I had, especially one year ago here," Sinner said. "I learned, and I had to accept to learn from myself, [from] my body language. I worked a lot on that."

The Italian was never in danger of an early upset this edition. His rise to No. 1 was guaranteed Wednesday just before he defeated Grigor Dimitrov to reach his first semi-final at the French Slam. While happy with the accomplishment after the match, he made clear his concentration was fully on the blockbuster clash in front of him against Alcaraz.

Sinner took a two-sets-to-one lead against his rival in the last four with flashes of brilliance and showed admirable determination when he battled through hand cramp to win the third set. The Spaniard ultimately rallied for a five-set victory and lifted the trophy on Sunday.

As much as the loss stung, the new World No. 1 had no time to dwell. He immediately turned his attention to the work ahead to prevent such a loss from happening in their next encounter.

"The winner is happy and then the loser tries to find a way to beat him the next time," Sinner said. "I think that's exciting. That's what I will try to do."

It was not a setback, but another step in Jannik's journey.