MELBOURNE, Australia — Novak Djokovic's hopes for a second straight Australian Open title died under a broiling sun Tuesday.

With ice packs and massages failing to provide relief, third-ranked Djokovic looked increasingly woozy and had to give up while trailing 6-7 (3), 6-4, 6-2, 2-1, allowing No. 7 Andy Roddick to claim a spot in the semifinals.

After losing 15 pounds with a tough offseason workout regimen under new coach Larry Stefanki, Roddick looked quicker and his backhand stronger. The match left little doubt about the American's stamina on a day when temperatures hit 95 degrees with not a cloud in the sky.

"It's rewarding to come out on a day like today, when it's pretty hot, and feel pretty good. That's what you do the work for," said Roddick, who will face the winner of the late match between second-ranked Roger Federer and No. 8 Juan Martin del Potro.

The heat was slowly rising but wasn't a major issue in the opening match, when Vera Zvonareva ran off 11 straight games in a 6-3, 6-0 win over 2007 Wimbledon finalist Marion Bartoli of France to reach the semifinals for the first time in 25 majors.

Seventh-ranked Zvonareva will meet the winner of an evening match with a compelling story line — third-ranked Dinara Safina of Russia against unseeded Jelena Dokic, an Australian making her return to a Grand Slam after a three-year absence due to personal problems.

The Australian Open has turned into a struggle for survival: Djokovic was the fourth player to quit in mid-match in two days.

The 21-year-old Serb lamented he didn't get to sleep until 6 a.m. Monday after his previous match against Marcos Baghdatis ended at 2:26 a.m. Monday, and that he had been unable to practice as a result. He said he had requested another night match.

"Didn't really have time to recover," Djokovic said. "Conditions were extreme today. It did affect more on me than him, as you could see. But, you know, that was the situation. I just have to cope with it. Really tried my best, but sometimes you can't fight against your own body."

He said he was cramping and sore.

"Obviously, it's very disappointing way to finish my first Grand Slam of the year," he said. "But you have to take the best out of it and be positive. There is still a long season in front of me."

It's not the first time Djokovic hasn't been able to finish a big match in a major. He retired in his quarterfinal against Rafael Nadal with a back problem at the 2006 French Open and his semifinal against the Spaniard at Wimbledon in 2007.

Both Djokovic and Roddick rode their powerful serves — Roddick cranked his up to 139 mph — and easily held until the first-set tiebreaker. Djokovic took control with one of his best stretches, whipping winners on the first four points, and a serve return that trickled over the net gave him set point.

Roddick dropped only two points in five service games in the second set, getting the first break of the match for a 4-3 edge. A series of drop shots made the difference. Djokovic tried a drop shot but Roddick got to the ball on the dead run, flicked a drop of his own and blocked Djokovic's return into the open court. On break point, Roddick sliced a drop that set up an overhead winner.

Djokovic already was wilting in the heat, draping towels packed with ice around his neck during changeovers. He lost his serve in the first game of the third set, double-faulting twice and was clearly laboring — trying to end points fast and lingering in the shade behind the baseline.

He did manage to break back in the second game with a perfect lob after drawing Roddick to the net. Djokovic held in the next game but looked increasingly weary and shrugged toward his coach. Even his fans' shouts annoyed him.

After the game, Djokovic called for the trainer, who massaged his thighs with ice. He draped a cold towel around his shoulders again.

"I was kind of just playing my side of the court and I didn't notice until the umpire said that they had someone coming out to see him," said Roddick, the 2003 U.S. Open champion. "I feel bad for Novak right now. He worked so hard for this last year. To not get a fair chance to defend his title, that's too bad."

By contrast, Roddick — who grew up in Texas and Florida and has said he loves the heat — looked fresh until the end, though the long timeout seemed to affect him briefly as he double-faulted three times while holding serve.

Roddick ended up winning the last five games of the set, taking the last 11 points. Djokovic barely moved for some serve returns and didn't get a ball back in play as Roddick held at love to finish the set.