Roger Federer's hopes of a 21st grand slam title are fading.
The tennis star said in a video on Instagram that he will need another knee surgery.
"I will be on crutches for many weeks and then also out of the game for many months," Federer said.
Federer, 40, said he injured his knee during the grass-court season. He was knocked out of Wimbledon in the quarterfinals and didn't play in the Olympics.
This year was supposed to be Federer's comeback. He was sidelined for much of 2020 after having one of his knees repaired.
He will miss the U.S. Open, where Novak Djokovic and Rafael Nadal will try to break a three-way tie for the man with the most grand slam titles.
Federer, Nadal, and Djokovic have 20 grand slam titles each.
Federer said the surgery will give him a "glimmer of hope" to return to the tour, but he's also realistic about his expectations.
"Don't get me wrong, I know how difficult it is at this age right now to do another surgery and try it," Federer said.
Following the U.S. Open, the next grand slam would be the Australian Open in 2022.