Roger Federer withdrew from the ATP Toronto and Cincinnati Masters events on Thursday, adding to fitness concerns and fears the 20-time Grand Slam champion might not 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 a knee injury.

Federer, whose last Slam title came at the 2018 Australian Open, pulled out of the last major tune-ups for the US Open.

The Toronto event begins on Monday and Cincinnati opens on August 16, two weeks before the Flushing Meadows fortnight begins for the year's final Grand Slam tournament.

Federer has not played at the US Open since reaching the 2019 quarter-finals, missing last year's event.

Arthur Ashe Stadium was the site of an epic run of five consecutive US Open titles for Federer from 2004 through 2008. He reached the final again in 2009 only to lose in five sets to Juan Martin del Potro.

Federer has reached the US Open final only once since then, losing to Djokovic in the 2015 championship match.

Federer, who has played five events this year, underwent two right knee operations in 2020 following the Australian Open before returning to ATP play at Doha in March.

Former world number one Federer, now ninth in the rankings, made his best run of the year to the last eight at Wimbledon, where he is an eight-time champion.

Federer, Rafael Nadal and top-ranked Novak Djokovic all have a men's record 20 career Grand Slam titles ahead of the US Open, where Djokovic will try to complete the first men's calendar-year Grand Slam since Rod Laver in 1969.

Federer owns two Toronto titles and a record seven trophies in Cincinnati.

The Toronto Masters also announced that Olympics champion Alexander Zverev, Italy's Matteo Berrettini, Canadian Milos Raonic and Spaniard Pablo Carreno Busta have also withdrawn.