Andy Roddick, left, talks with Novak Djokovic after Djokovic retired from their match at the Australian Open today.

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

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.

He will face second-ranked Roger Federer, who moved within two victories of his record-tying 14th Grand Slam by routing No. 8 Juan Martin del Potro of Argentina 6-3, 6-0, 6-0. Federer, who owns a 15-2 advantage in previous matches with Roddick, ran off the last 13 games against del Potro.

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.

While the crowd during the night match was definitely pro-Federer, with Swiss flags abounding, it began backing the increasingly dispirited del Potro. But nothing could stop Federer, who won 51 of the last 65 points and finished with 38 winners to a mere nine unforced errors.

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 fellow Russian Dinara Safina, who survived 11 double-faults and 36 unforced errors to beat Australia's Jelena Dokic 6-4, 4-6, 6-4 in a back-and-forth match with 11 service breaks. It's the second straight Grand Slam semifinal for Safina — younger sister of 2005 men's champion Marat Safin — who lost to eventual champion Serena Williams at the U.S. Open.

Dokic's loss ended one of the tournament's most compelling stories: The former Wimbledon semifinalist was making her return to a Grand Slam after a three-year absence due to personal problems.

Safina apologized to the crowd for beating the local favorite, who advanced through a wild-card play tournament and was ranked No. 187.

The Australian Open has turned into a struggle for survival: Djokovic was the fourth player to quit in midmatch 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. ... But, you know, that was the situation. I just have to cope with it."