Three years ago at Wimbledon, John Isner took part in the longest match in tennis history. On Wednesday at the All England Club, he was hardly on the court long enough to break a sweat.The 18th-seeded American pulled out in the third game of his second-round match against Adrian Mannarino with a left knee injury.Isner, who was trying to make the third round of Wimbledon for the first time, said he landed awkwardly after serving the third point of the match."It didn't pop," Isner said. "It just grabbed, like, really badly, and I knew I was in serious trouble then. I mean, I knew at that point it was not likely I was going to be able to play."During the second game, Isner called for a medical timeout and had his knee wrapped, but when he resumed, he was clearly hobbled and unable to track down the ball. He lost the second game and after the first point in the third, he decided to stop.The retirement came three years after Isner won the longest match in tennis history, beating Nicolas Mahut 70-68 in the fifth set in a Wimbledon match that lasted more than 11 hours over three days.Earlier this year, Isner pulled out of the Australian Open with a right knee injury.He was scheduled for more tests later Wednesday to determine the extent of the injury."I just can't bend my knee," Isner said about 30 minutes after leaving the court. "I can walk as long as I keep it straight. So it's just putting any sort of weight. I couldn't even bend the slightest, and I still can't."