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.
"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."
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.
Pennetta saved four match points at 6-5 down in the second set against Zvonareva and then two more in the ensuing tie-break after which the Russian went to pieces.
Injury fears follow Nadal into last 16 at US Open
Meanwhile, Rafael Nadal´s bid to complete a career Grand Slam hit a potential snag Sunday when a right abdominal muscle injury brought new health concerns even as the Spaniard reached the last 16 at the US Open.
Third seed Nadal received treatment from a trainer in the third set of a 7-5, 6-4, 6-4 victory over 32nd-ranked countryman Nicolas Almagro and his reluctance to speak about it after the match added more fitness worries.
"I don´t want to talk about injuries," Nadal said. "I try my best every day. I won the match in three sets."
Pressed on whether or not the treatment helped him in the third set, the six-time Grand Slam winner replied, "I won 6-4. You can see I didn´t serve very fast but I think I played better from the baseline than before."
British second seed Andy Murray, last year´s US Open runner-up, defeated US 195th-ranked US wildcard Taylor Dent 6-3, 6-2, 6-2 in the feature night match at Arthur Ashe Stadium.
"I love playing night matches," Murray said. "The atmosphere was great. Obviously the crowd wanted Taylor to win but I played very well."
The 22-year-old Scotsman, trying to become the first British man to win a Slam title since Fred Perry in 1936, will play for a quarter-final spot against Croatian 16th seed Marin Cilic, who beat Uzbek Denis Istomin 6-1, 6-4, 6-3.
"He´s got a big serve and moves well for his height," Murray said of Cilic. "I´ll need to return well and I´ll have to be very solid from the back."
Nadal, whose best US Open run came into last year´s semi-finals, will play for a quarter-final berth against French 13th seed Gael Monfils, who was ahead 6-3, 6-4, 1-0 when Argentina´s Jose Acasuso retired with left knee pain.
"I have to come out ready for the next match," Nadal said. "For today it was enough but for sure I can do better."
Nadal called the trainer after breaking to pull within 2-1 in the third set.
After laying on his back on the concrete for treatment of what could be a flare-up of a muscle strain suffered last month at Cincinnati, Nadal held serve, broke for a 3-2 edge and served out to win despite discomfort.
"I feel it a little bit now but I will try to recover in time for my next match," Nadal said moments after the final point.
Nadal, the reigning Australian Open champion, is coming off a two-month layoff due to knee tendinitis, a break that kept him from defending his 2008 Wimbledon crown.
"I just plan to keep on improving my tennis and keep playing this tournament because it´s special to me," Nadal said. "It´s special motivation when you try to come back and play your best as fast as possible."
Murray, who matched Novak Djokovic with a tour-best 56th season triumph, fired 39 winners and seven aces against only 13 unforced errors and overpowered Dent, back at the elite level after missing two years for major back surgery.
"I returned really well. That was probably the key to the match," Murray said. "I managed to finish the match off well at the end."
With Murray´s victory, a US Open record 14 of the top 16 seeds reached the fourth round, breaking the old mark from 1992. All 16 men´s top seeds had reached the third round, a Grand Slam first.
The only absentees in the round of 16 were US fifth seed Andy Roddick, who lost Saturday, and French ninth seed Gilles Simon, forced to retire with a sore right knee with Spaniard Juan Carlos Ferrero leading 1-6, 6-4, 7-6 (7/5), 1-0.
Ferrero, the 2003 French Open champion and 2003 US Open runner-up, won his first title in six years at Casablanca in April and will face Argentine sixth seed Juan Martin del Potro for a last-eight berth.
Del Potro lost six games in a row at one stage but fired 20 aces in beating Austrian Daniel Koellerer 6-1, 3-6, 6-3, 6-3.
No Frenchman has won a Grand Slam singles crown since Yannick Noah at the 1983 French Open, but seventh seed Jo-Wilfried Tsonga, last year´s Australian Open runner-up, and Monfils are still in the hunt to end the drought.
Tsonga ousted compatriot Julien Benneteau 7-6 (7/4), 6-2, 6-4 to book a date with French Open semi-finalist and 2007 Australian Open runner-up Fernando Gonzalez, the Chilean 11th seed who beat Czech Tomas Berdych 7-5, 6-4, 6-4.
"(Tsonga) serves really big and he has strong shots," Gonzalez said. "We have to see who can make more winners."
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