Roger Federer withdrew from the ATP Cincinnati Masters on Thursday, adding to fitness concerns the 20-time Grand Slam champion might not be able to play in the US Open.
The Swiss legend, who turns 40 on Sunday, reached the Wimbledon quarter-finals but skipped the Tokyo Olympics due to the knee injury.
Federer, whose last Slam title came at the 2018 Australian Open, pulled out of the last major tuneup for the US Open.
The Cincinnati event opens August 16, two weeks before the start of the US Open in New York.
