NEW YORK: Comeback queen Kim Clijsters split the Williams sisters on Sunday when she sent Venus reelling out of the US Open.

The 26-year-old Belgian, who has been the talk of the tournament as she continues her return from two and a half years out of the game to marry and have a baby, won a roller-coaster of a match 6-0, 0-6, 6-4.

She will play China's Li Na in the last eight with a possible tilt at defending champion and tournament favourite Serena Williams awaiting in the semi-finals should she win that.

"It was such a weird match especially the first two sets. When I lost the second set I said, 'Just start again and fight for every point,'" she said.

"I've been working really hard the last seven or eight months and I am enjoying it and that is important.

"Tennis is a great sport, but I am just happy that we have a family and I can balance both."

Serena, meanwhile, was all business as she dispatched Slovakia's Daniella Hantuchova 6-0, 6-2 in just 64 minutes and will next play Italy's Flavia Pennetta who saved six match points against Russian seventh seed Vera Zvonareva before winning 3-6, 7-6 (8/6), 6-0.

The Clijsters-Venus Williams tie was a game of three distinct sets with Clijsters blanking Williams in the first and the American returning the favour in the second.

The decider finally saw both women playing well at the same time and it produced a high quality drama that turned on a dropped serve by Williams in the third game as she double-faulted on break point.

The Belgian, bidding to become just the third mother to win a Grand Slam title after Australians Evonne Goolagong and Margaret Court, got to 5-3 ahead with Williams having to serve to stay in the match.

The American successfully did that but Clijsters served out for the match, erasing two break points in the process.

It was a routine win for tournament favourite Serena in stark contrast to the trials and tribulations of several of her main contenders, especially the Russian brigade of top seed Dinara Safina, Maria Sharapova and Elene Dementieva who were all upset

The 27-year-old title-holder broke serve in the sixth and eighth games of the first set against the Slovak, a Grand Slam regular who struggles against the top players.

The second set was one-way traffic as the title-holder made it 10 games in a row to march into the semi-finals in just 64 minutes.

In four matches, she has yet to drop a set and has lost just a total of 17 games.

"I played well and stayed focussed," she said. "I traditionally play really well in fourth-round matches so I want to to keep this level and stay focussed for my next match.

"Everyone is playing so well and some of the seeds have been struggling all summer and you can't underestimate anyone."

With the win, Williams took her Grand Slam record for the year to 22-1, her only loss being to Svetlana Kuznetsova in the French Open quarter-finals. She won both the Australian Open and Wimbledon crowns taking her career Grand Slam total to 11.

Li's comprehensive 6-2, 6-3 win over Italy's Francesca Schiavone is only the third time a Chinese player has reached the quarter-finals of a Grand Slam tournament.

Li herself was the first at Wimbledon 2006, where she lost to Clijsters, followed by Zheng Jie's wildcard run into the 2008 Wimbledon semi-finals.

"Everything went perfect today," the 27-year-old 18th seed said.

"This was my dream to get to the quarters here and I hope I can win three more matches."