Injuries have knocked two players out of Wimbledon — the Belgian who stunned Rafael Nadal in the first round and the American who won the longest match in tennis history here three years ago.

Steve Darcis, who beat two-time champion Nadal in straight sets on Monday in one of Wimbledon's greatest upsets, pulled out on Wednesday because of a right shoulder injury.

The 135th-ranked Darcis withdrew just hours before he was scheduled to play Lukasz Kubot of Poland.

John Isner, the 18th-seeded American, retired during his second-round match against Adrian Mannarino of France with a left knee injury after only two games.

Isner took a medical time out during the second game and a trainer taped the knee. But he was clearly hobbled and decided to quit after serving the first point in the third game.

The retirement came three years after Isner beat Nicolas Mahut 70-68 in the fifth set in a match that lasted more than 11 hours over three days.

Second-seeded Victoria Azarenka has pulled out of Wimbledon after hurting her right knee in her opening-round win.

Early in the second set of her match against Maria Joao Koehler, Azarenka did the splits near the baseline, then crumpled to the grass, clutching her right knee and sobbing.

Azarenka recovered after a medical timeout to finish out the 6-1, 6-2 victory on Monday but she moved gingerly after falling and said she wasn't sure about the extent of the injury.

On Wednesday, minutes before her second-round match against Flavia Pennetta was to begin, she pulled out of the match.

Considered one of Serena Williams' toughest matchups, Azarenka has won the last two Australian Opens and had reached the semifinals in her last two Wimbledon appearances.