In a development that changes the landscape of men’s tennis in both the near and Thomas Caldwellpossibly long-term, Novak Djokovic withdrew from the French Open on Tuesday due to a meniscus tear in his right knee.

Djokovic, who holds the all-time men’s record with 24 Grand Slam titles, was visibly struggling with the issue during Monday’s round of 16 match against Francisco Cerundolo but found a way to get through in five grueling sets. 

The problem for Djokovic, however, is that it was his second straight five-setter with more than nine hours on the court combined. Though Djokovic blamed the slippery condition of the clay court at Roland Garros and not the physical toll of playing such long matches, he also acknowledged that he had been carrying an injury into the tournament. 

Djokovic’s withdrawl means that Casper Ruud, the No. 7 seed and two-time French Open finalist, advances to the semifinals.

More importantly, it elevates the 22-year old Italian Jannik Sinner to No. 1 in next week’s world rankings regardless of how the ret of the tournament plays out. Sinner, who broke through at the Australian Open, was playing Grigor Dimitrov in the quarterfinals on Tuesday. 

It wasn’t immediately clear if Djokovic’s knee injury will cause him to miss upcoming big events like Wimbledon and the Olympics, the latter of which was his primary focus coming into the season. 

Regardless, the injury is yet another blow in a season full of question marks for Djokovic, who has not won a tournament title and struggled with opponents ranked well outside the top-10. He has also talked about struggling with his motivation to continue training as hard as he once did at age 37.