MELBOURNE: Defending champion Serena Williams survived against Victoria Azarenka in a classic comeback win Wednesday, preventing back-to-back upsets after her sister Venus lost to Li Na.

Venus Williams had a chance to serve for the match in the second set against Li but she went down in a flurry of errors.

Serena Williams lost the first set and was down two breaks of serve against Azarenka until she rallied to win five straight games to get herself back on track for a 4-6, 7-6 (4), 6-2 win.

"I'm surprised. I didn't expect to win when I was down 0-4. I was like, 'Oh well, at least I'm still in doubles,'" said Williams, who admitted she'd even started thinking about the trip home.

"I was actually thinking — it wasn't a champion's thought — if I lose today and I don't win in doubles, I guess I can catch a flight on Friday.

"That's not what you're supposed to think but that's what I was thinking."

Azarenka wilted in their quarterfinal here last year, retiring in the second set with a virus after taking the first set off Serena Williams.

This time, the 20-year-old Belorussian was undone by a trademark comeback from Williams, who is the only player to save match points on the way to three Grand Slam titles. She did it at Wimbledon last year and at the 2003 and 2005 Australian Opens.

The 11-time Grand Slam champion has rallied when the odds are seemingly stacked against her, which has helped Williams win the Australian title each odd-numbered year since beating sister Venus in the 2003 final.

Now, instead of the projected sisters semifinal on the top half of the draw, it will be: Serena Williams vs. No. 16 Li of China. Serena's close finish was only 18 hours after a knee problem forced men's defending champion Rafael Nadal to quit his quarterfinal against Andy Murray while he trailed in the third set.

Li rallied from a set and a break down, breaking Venus Williams' serve to stay in the match in the second set, on the way to her 2-6, 7-6 (4), 7-5 triumph.

Azarenka looked as if she'd continue the Williams family woes. She broke the four-time Australian Open champion Serena in the opening game and then, after fending off three break points, held her own opening serve in a game that lasted more than 15 minutes and went to deuce nine times.

It was mostly one-way until she led 6-4, 4-0, then Williams started her comeback — winning five straight games and then getting on top again at the end of the tiebreaker.

Williams dominated the third set, converting both her breakpoint chances.

That's more than can be said of her older sister, a seven-time Grand Slam champion, who made a pile of unforced errors against a player who had never gone past the quarterfinals at a major.

Li's progress to the semifinals gave China two players into the last four of a tennis major for the first time.

"Best day of my whole life," Li said, grinning widely.

Zheng Jie will play a semifinal against former No. 1-ranked Justine Henin in the other half of the draw.

"Good for both players. Also good for China's tennis," Li said. "Also good for the fans. Thank you."

It was another disappointing exit for the 29-year-old Williams, who bristled at the suggestion that her only chance at another major was at Wimbledon.

Venus Williams has won five Grand Slam singles crowns on grass and been in another three finals at Wimbledon. She hasn't won a major on any other surface since she successfully defended her U.S. Open title in 2001. That doesn't stop her from trying.

"Well, like I said so many times before, if I would have, kind of gotten involved in what people said I would have never left the ghetto," Williams said. "So here I am playing pro tennis, playing well. The sky's the limit in this sport. I'm looking for that."

The experts picked a Williams sisters semifinal. And when Venus was serving for the match at 5-4 in the second set, it still seemed likely.

But Li rallied, breaking Williams in that game and eventually forcing and then dominating a tiebreaker.

Williams held to open the third set, which then featured six consecutive service breaks until Li held to even it at 4-4. She broke Williams and then had her own chance to serve for the match, but didn't win a point in that game.

Williams returned the favor, losing four straight points on her serve to give Li a second chance to serve it out from 6-5.

Li, who had never gone beyond the quarterfinals at a Grand Slam, needed three match points, eventually converting it with consecutive forehand winners down the line.

The match featured a combined 110 unforced errors, Williams committing 53 of those. Williams had 11 double-faults — five in the third set — against Li's five in the match.

The pair hit 27 winners each.

Li survived two match points in her third-round win over Hungary's Agnes Szavay, now she's gone further than ever at a major and is talking about a top 10 ranking.

She said she might even celebrate with a beer.

"In China we say if you have tough time and then you return back, maybe have good luck for after," she said. "So I still believe that, yeah."

Li beat Venus in the quarterfinals of the Beijing Olympics in their only previous meeting but was on the back foot early this time.

Williams started convincingly, leading 4-0 in the first, before Li held and then broke Williams.

Williams won the next four games, clinching the first set and going up a break in the second, before Li rallied again.

Being so close to defeat stirred Li into action.

"I just played my tennis. I know I lose the first set but thanks for the coach, never give up," she said.

"He was teaching me a lot. I know sometimes talk too much but nice guy," she added, laughing.

Nadal, who won last year's final in five sets over Roger Federer, was behind 6-3, 7-6 (2), 3-0 when he withdrew Tuesday night against No. 5 Murray because he didn't want to risk long-term damage to his knee.

Andy Roddick also needed a medical timeout — after the first set of his earlier match against No. 14 Marin Cilic. His sore right shoulder was making his fingers numb.

Cilic eventually advanced 7-6 (4), 6-3, 3-6, 2-6, 6-3.

Federer is still in the equation at Melbourne Park, where he plays Nikolay Davydenko in the quarterfinals Wednesday. Novak Djokovic and Jo-Wilfried Tsonga meet in a rematch of the 2008 final.