MELBOURNE, Australia — Defending champion Novak Djokovic struggled in the heat Tuesday before retiring from his Australian Open quarterfinal against American Andy Roddick.
Roddick was leading 6-7 (3), 6-4, 6-2, 2-1 when the third-seeded Djokovic quit, maintaining Roddick's streak of semifinal appearances at Melbourne Park every alternate year since 2003.
Djokovic had just had his serve broken to go down 2-1 in the fourth set when he went to the side of the court and told umpire Carlos Ramos he could not continue.
He shook hands with Roddick and left Rod Laver Arena with the trainers.
The outside temperature with the roof open at Rod Laver Arena during the match was as high as 95.
He'd had a medical timeout after recovering a break to lead 2-1 in the third set and complained of heat-related problems.
Roddick, the 2003 U.S. Open champion, won the next five games to finish off the third set, winning the last 11 points.
Djokovic, who beat Roddick in the quarterfinals at last year's U.S. Open, finally held the opening game of the fourth set after nine minutes but was visibly slowing down and irritated.
"To be honest I didn't know (he was sick) until he called for help there. That's disappointing," Roddick said. "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."
Djokovic beat Roger Federer in the 2008 semifinals and collected his first major title by beating France's Jo-Wilfried Tsonga in the final.
That was the only title in the last 15 Grand Slams not won by either Federer or top-ranked Rafael Nadal.
Djokovic went out in the semifinals at the French and U.S. Opens and the second round at Wimbledon. He finished the year off by winning the season-ending Masters Cup.
The 21-year-old Serb has a history of retiring in big matches at majors. He failed to finish his quarterfinal against Nadal at the 2006 French Open and his semifinal against the Spaniard at Wimbledon in 2007.
Roddick will next play either No. 2 Roger Federer or No. 8 Juan Martin del Potro, who were to meet in a night match.
Federer is seeking a record-tying 14th Grand Slam singles title.
In the opening match in Rod Laver Arena, 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.
The 24-year-old Russian's best performance in six previous trips to Melbourne was the fourth round — she'd gone out in the first round at the Australian Open three times, including last year. She also reached the quarterfinals at the 2003 French Open.
Seventh-seeded Zvonareva rallied from an opening service break to dominate Bartoli in the remainder of the match.
"I'm very excited about it," said Zvonareva, who cut her unforced errors from 15 in the first set to two in the second. "I think it was a great day for me."
She's had four 6-0 sets out of the 10 in her five straight-sets wins.
"I'm not really thinking about the scores or sets or any statistics," she said. "I'm just trying to concentrate on every match and trying my best. And I think I've been doing pretty good so far."
Bartoli, seeded 16th, had ousted top-ranked Jelena Jankovic in the fourth round.
"I think she played just unbelievably well," Bartoli said of the last 11 games. "She barely missed one ball after that. I was hitting as hard as I could. She was always coming back with some better shots."