MELBOURNE, Australia (AP):

NOVAK DJOKOVIC'S hopes for a second straight Australian Open title ended 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 semi-finals.

The 26-year-old American 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, but lost their most recent meeting at Miami last April, ran off the last 13 games.

Playing andy always nice

"Playing Andy is always nice," said Federer, 27. "We've had some big matches over the years and it's always a pleasure to play against him because he brings energy to the court with his serve and his character. It's nice to play somebody my age. Everyone's so young now."

Djokovic said he was cramping and sore, and "didn't really have time to recover" from his previous match, which ended at 2:26 a.m. on Monday (AEST).

"Conditions were extreme today. It did affect more on me than him, as you could see," Djokovic said. "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."

Roddick, meanwhile, looked as fit as ever. After losing 15 pounds (seven kilogrammes) with a tough offseason workout regimen under new coach Larry Stefanki, he was quicker and his backhand stronger. The match left little doubt about the American's stamina on a day when temperatures hit 35C (95F) 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," Roddick said.

Nothing worked for del Potro against Federer, who won 51 of the last 65 points and finished with 38 winners to a mere nine unforced errors.

Everything was working for Federer, and his mix of speeds and spins was masterful. One service game in the first set summed it up - ace, backhand winner, forehand winner, volley winner, all in less than two minutes. As he served for the second set, a fan shouted: "You're perfect, Roger!"

Not quite, but very, very good.

"I thought it was going to be a tough match in the beginning," Federer said. "We had some tough rallies, and it really showed the direction of the match, I thought. I was very mistaken. I'm happy about that.

"The longer the match went, the more he struggled and the better I got. The last couple games ... You want to almost put him out of his misery because you know how tough it is for him."

Needs for improvement

Del Potro called it a bad day that showed he needs to improve to compete against the best.

"I can't do nothing in the match," del Potro said. "He play like No. 1 of the world."

In the first match of the day, Vera Zvonareva ran off 11 straight games to beat 2007 Wimbledon finalist Marion Bartoli of France 6-3, 6-0 to reach the semi-finals 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. It's the second straight Grand Slam semi-final for Safina - younger sister of 2005 men's champion Marat Safin - who lost to eventual champion Serena Williams at the US Open.

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

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

"I hope that next time you'll be behind me," she said.

Dokic was happy with her performance.

"There's nothing to be disappointed about," Dokic said. "It's been a great start to 2009. I couldn't have asked for anything more."

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

"Obviously, it's very disap-pointing 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."