The world No 1 has not been seen on court since beating Alexander Zverev to defend his Wimbledon title in July.
Sinner pulled out of the National Bank Open in Montreal last week and will now not feature in Cincinnati, raising concerns over his fitness ahead of the upcoming US Open, which starts on August 30, live on Sky Sports.
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"After consulting with my doctors and my team, I have to announce that I need to withdraw from the Masters 1000 tournament in Cincinnati," Sinner said.
"My 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.
"I'm very disappointed not to be able to play in Cincinnati and wish Bob [Moran] and his whole team a great and successful event.
"I can't wait to be back next year and I'm now focusing on getting ready for the US Open."
Injury has robbed Sinner of the chance of avenging his defeat to Carlos Alcaraz in the 2025 final, in which he was forced to retire injured five games into the first set.
The Cincinnati Open starts on Tuesday, also live on Sky Sports, with the tournament culminating with the singles championship finals on August 23.
Watch the ATP and WTA Tours and US Open from August 30, live on Sky Sports or stream with NOW and the Sky Sports app, giving Sky Sports customers access to over 50 per cent more live sport this year at no extra cost. Find out more here.
