For 121 days, Novak Djokovic remained untouched, not experiencing a single loss. Following his intense five-set battle against Spanish sensation Carlos Alcaraz in the Wimbledon final, the Serbian icon went on to secure consecutive victories in 19 matches.  

Regardless of Djokovic's form or mindset, his opponents struggled to make any impact on him. It had become a question of not whether Djokovic will triumph, but rather by what margin he will emerge victorious. 

However, late on Tuesday night, an Italian protagonist defied the odds and accomplished the seemingly impossible feat. With a boisterous home crowd as witness, Jannik Sinner engaged in an extraordinary battle against Djokovic at the ATP Finals in Turin. 

In a thrilling and exhausted encounter that lasted over three hours, the 22-year-old managed to prevail, securing a monumental victory of 7-5, 6-7 (5), 7-6 (2). This triumph marked Sinner's first-ever career win over the 24-time Grand Slam champion and current world No. 1. 

In a closely contested clash where both players won an equal number of points, only three breaks of serve occurred throughout the entire match. Sinner managed to secure a crucial late break at five-all, ultimately claiming the first set. Breaks of serve were exchanged by both players in the middle of the third set. However, when it came down to the third set tie-break, Sinner elevated his game to another level.  

The Italian displayed immense composure and unleashed powerful groundstrokes under huge pressure. He surged ahead with an impressive run of five consecutive points, establishing a lead that became insurmountable for Djokovic. Sinner never relinquished his advantage thereafter, claiming a famous scalp.

"That was a really high-level match," Sinner said of his hard-fought win. "I think I was really brave and intelligent in important moments, especially the third set. I felt like we were both serving really well, so there were not so many exchanges, but when the exchanges were played we both played really well... When you win against the world No. 1 and he won 24 Grand Slams, It means a lot to me."

Among the young rising tennis stars, both Sinner and Alcaraz have hinted at the possibility of challenging Djokovic in the long run. However, it is worth noting that they have not simultaneously reached their full potential. Sinner had a slow start to the season, while Alcaraz has struggled to maintain his form since Wimbledon. 

As for Djokovic, this shock defeat has placed him in a precarious position, teetering on the edge of elimination from the season-ending ATP Finals. A loss to Hubert Hurkacz on Thursday would spell the end of Djokovic's run in the tournament. 

However, the ever-dependable Serb still has a chance to turn things around. If he manages to win three consecutive matches, he could still win the tournament, silencing any doubts and reminding everyone of his unparalleled dominance. Tuesday night's loss could then be dismissed as nothing more than a minor blip.