Roger Federer has withdrawn from this month's Toronto and Cincinnati Masters events with a knee injury - putting his US Open participation in doubt.
The 39-year-old missed the Tokyo Olympics due to a "setback to his knee" after being knocked out of the Wimbledon quarter-finals.
Federer, a 20-time Grand Slam champion, had double knee surgery last year.
The Toronto Masters starts on 9 August, Cincinnati is a week later and the US Open begins on 30 August, in New York.
Federer - who turns 40 on Sunday - has only played 13 matches in 2021 after just six matches in the pandemic-hit 2020 season.