Roger Federer has withdrawn from this month's Toronto and Cincinnati Masters events with a knee injury - putting his US Open participation in doubt.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.

The Swiss won the last of his five US Opens in 2008 and his most recent major was the 2018 Australian Open.

Since then Novak Djokovic has claimed eight more majors and Rafael Nadal has picked up four. All three great rivals now stand level on 20 Grand Slams.

Federer, ranked ninth in the world, has 103 career titles from his 23 years on tour, with a combined 310 weeks at number one.

Tokyo men's singles gold medallist Alexander Zverev will also skip the tournament in Toronto to get some extra rest.

Meanwhile, Nadal, 35, was knocked out of the Citi Open in Washington in the third round by South Africa's Lloyd Harris - his comeback event after a two-month lay-off with a foot injury.

Harris will face Japan's Kei Nishikori in the quarter-final after Nishikori beat Britain's Cameron Norrie 3-6 6-3 6-3.