Taylor Fritz revealed that a flare-up of his long-standing right knee problem caught him completely by surprise during his Wimbledon quarterfinal defeat to Alexander Zverev on Wednesday.

The American entered the match having won his previous seven meetings against the German but struggled with severe knee pain early in the contest. After receiving medical treatment at the start of the second set, Fritz went on to lose 6-4, 6-4, 6-2.

"I started feeling pain in my knee just a few games into the match," Fritz said after the loss. "I still believe I could have played much better despite the injury, but Zverev was serving incredibly well and hitting the ball with great confidence."

Fritz admitted the injury was unexpected, especially after arriving at Wimbledon in strong form following runner-up finishes in Stuttgart and Halle, where he defeated Zverev in the semifinals.

The 28-year-old said he briefly panicked when the pain returned, recalling only minor discomfort near the end of his previous match against Alexander Bublik. After taking a light practice day before the quarterfinal, he expected to feel fully recovered.

"I felt great during the warm-up, so I really don't have an explanation for why it happened after just three games," he said.

Zverev now advances to the Wimbledon semifinals, where he will face Britain's Arthur Fery. Despite the expected home support for Fery, Fritz believes the German will be difficult to stop.

"Zverev is an extremely tough opponent, especially because of his serve," Fritz said. "He consistently lands a high percentage of first serves with incredible pace, and he's playing with a lot of confidence. It should be a very interesting match."