he spectators were still settling into their seats when Juan Martin del Potro chased a David Ferrer overhead into the corner, only to lose traction on the slick grass.

Del Potro's knee straightened suddenly, then bent backward. The tall Argentine crumpled to the ground and barrel-rolled twice into the far edge of Centre Court.

Only five points into the Wimbledon quarterfinal, the match appeared over before it had really gotten going.

No such luck for Ferrrer.

First at a limp, then at a jog, then at a sprint, the eighth-seeded del Potro recovered for a 6-2, 6-4, 7-6 (5) victory over the fourth-seeded Ferrer to make his first Wimbledon semifinal. His opponent will be top-seeded Novak Djokovic, whose 7-6 (5), 6-4, 6-3 victory over No. 7 Tomas Berdych ended less than five seconds after del Potro hit his final shot.

"It's really, really painful," del Potro said of the injury that has been bothering him since he first slipped on the Wimbledon turf last Saturday. "I twist my knee once again, but the doctor gave me some magic pills. I could finish the match. I'm so glad to go through."

During a tournament with more than its share of twists and turns, Djokovic and del Potro have gone through virtually unscathed — on the scoreboard at least. Neither man has dropped a set.

Djokovic overcame a two-break, 3-0 deficit in the second set to cruise through his latest victory. He's in his 13th straight Grand Slam semifinal and in search of his seventh major title.

Del Potro, meanwhile, is starting to show the form he used to win his only major, the U.S. Open in 2009. He's 3-8 lifetime against Djokovic, thought one of those victories came at the All England Club when he beat the Serb in the Olympic bronze-medal match last year. Djokovic also won their last meeting, earlier this year on hardcourt in Indian Wells, California.

Before breaking down that matchup, though, del Potro had an appointment with an ice tub and his trainers. His knee, mummified by athletic tape even before he went down in Wednesday's match, needs work. But if the doctors can patch him up, he's certainly playing some of his best tennis.

"In the end, I played my best forehand ever this Wimbledon," del Potro said.

Djokovic has been feeling pretty good about his game, as well.

"I've been playing my best tennis of the year on grass," he said. "I'm glad because it's not my most preferred surface. Hopefully, my experience can help me."

After their matches ended, the next two quarterfinals began — one pitting second-seeded Andy Murray of Britain against Fernando Verdasco of Spain, the other an all-Polish affair, with Lukasz Kubot going against No. 24 Jerzy Janowicz, with the winner to become the first Polish semifinalist in Grand Slam history.