Jannik Sinner's US Open preparations have suffered another setback after the world No. 1 withdrew from the Cincinnati Open with a right knee injury, leaving his return to competitive tennis uncertain with the hard-court Grand Slam just weeks away.

Sinner, who has not played since successfully defending his

Wimbledon title last month, announced his withdrawal from the Masters 1000 event on Sunday after also pulling out of the tournament in Toronto.

The 24-year-old Italian said he was not yet ready to compete despite working with his medical team and will now focus on recovering in time for the US Open, which begins on August 30.

âAfter consulting with my doctors and my team, I have to announce that I need to withdraw from the Masters 1000 tournament in Cincinnati,â Sinner said in a statement released by tournament officials.

âMy right knee has been bothering me and even though we have been working hard with my medical team, I have to accept that I'm not ready to compete yet. Iâm very disappointed not to be able to play in Cincinnati. I can't wait to be back next year and I'm now focusing on getting ready for the US Open,â the statement added.

Sinner, Alcaraz absent from Cincinnati

Sinner's withdrawal comes just days after Carlos Alcaraz pulled out of Cincinnati with a lingering right wrist injury.

The back-to-back withdrawals mean the tournament will be without the world's top two men's players when the singles competition begins.

The timing is particularly significant for Sinner. Cincinnati has traditionally been one of the final major hard-court tune-ups before the US Open and was the tournament where he triumphed two years ago before going on to win the New York Grand Slam.

Sinner enters the US Open as one of the leading contenders after winning his fifth Grand Slam title at Wimbledon. He needs only the French Open to complete a career Grand Slam.

Alcaraz also racing against time

Alcaraz, meanwhile, has been sidelined since April after suffering the wrist injury and missed the French Open and entire grass-court season, including Wimbledon.

The Spaniard was expected to return in Cincinnati as he prepares to defend his US Open title. Recent social media images and videos showed the seven-time Grand Slam champion practising on court, raising hopes that he would be ready for the hard-court swing.

With both Sinner and Alcaraz now missing Cincinnati, their fitness will remain a major talking point heading into the final stretch before the US Open.