MELBOURNE, Australia — Venus Williams was one service game away from holding up her end of a projected sisters semifinal at the Australian Open, until she stumbled in a flurry of errors and lost to No. 16 Li Na.

Li reached the Grand Slam semifinals for the first time with the 2-6, 7-6 (4), 7-5 quarterfinal win on Wednesday, giving 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 was playing 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."

The experts expected Venus Williams to beat Li and set up a semifinal with her sister and defending champion Serena Williams, who was playing her quarterfinal in the following match on Rod Laver Arena.

And when the seven-time major winner was serving for the match at 5-4 in the second, it seemed a formality.

But Li rallied, breaking Williams in that game and eventually forcing and then dominating a tiebreaker against the former No. 1-ranked American.

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 at 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.

Serena Williams, who has won the Australian Open title each odd-numbered year since beating her sister in the 2003 final, was playing Victoria Azarenka in the quarterfinals.

Li had beaten 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 her 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.

Rafael Nadal, who won last year's final in five sets over Roger Federer, withdrew because of an injured knee while trailing Britain's Andy Murray 6-3, 7-6 (2), 3-0 in Tuesday's quarterfinals.

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.

Roddick played on for another four sets. But, like Nadal, he was gone in the quarterfinals. Cilic advanced 7-6 (4), 6-3, 3-6, 2-6, 6-3 and will meet Murray in the semifinals.

Nadal tweaked his knee in the 11th game of a second set that had already been interrupted for nine minutes by fireworks related to Melbourne's Australia Day celebrations.

Nadal's fall sent a murmur around the stadium, but he eased concern by playing a shot by instinct as he sat near the baseline.

He didn't show any outward signs of being hurt until calling for a medical timeout while serving at 0-1 and 15-all in the third set. He took a three-minute break while the trainer worked on his right knee. He played only 13 more points before quitting.

Murray was convinced he would have beaten Nadal anyway.

"When the big moments came in the match, I thought I dictated what happened," Murray said. "From my side, I played really well and deserved to be up when the match was stopped."

Nadal said he didn't want to risk long-term damage to the knee and being sidelined for long periods like he was last year, when he was unable to defend his Wimbledon title.

Federer won Wimbledon and 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.

Former No. 1 Justine Henin resumed her strong comeback from retirement by beating Nadia Petrova 7-6 (3), 7-5 to reach the semifinals. She next plays 2008 Wimbledon semifinalist Zheng Jie, who beat Maria Kirilenko 6-1, 6-3 in the other quarterfinal.