Cibulkova upset 2004 champion Svetlana Kuznetsova 7-5, 7-6 (4) in the fourth round Monday. Monfils held off two set points in the third to defeat fellow Frenchman Richard Gasquet 6-4, 7-5, 7-5.

Cibulkova is the lowest-ranked player left in the women's draw. The 45th-ranked Slovakian was a semifinalist at last year's French Open but had never been past the third round at Flushing Meadows.

Third-seeded Novak Djokovic rolled into his sixth straight Grand Slam quarterfinal, defeating Mardy Fish in straight sets.

Djokovic beat the 19th-seeded Fish 6-3, 6-4, 6-1 on Monday. Djokovic converted 6 of 8 break points.

Djokovic, the 2008 Australian Open champ from Serbia, has reached at least the semifinals at Flushing Meadows the last three years.

Fish was making his deepest run at a Grand Slam this year after failing to advance past the second round at the first three majors. Djokovic improved to 6-0 against Fish.

Kuznetsova was the two-time Grand Slam champion in Monday's match but didn't look it. The 11th-seeded Russian had 10 double-faults and 42 unforced errors. Kuznetsova reached a second US Open final in 2007 but hasn't made it beyond the fourth round since.

"I think she plays good, but my level is higher, and I have to win these matches," Kuznetsova said.

The last three women's quarterfinalists were to be decided later Monday in matches pitting No. 1 Caroline Wozniacki against No. 14 Maria Sharapova; Andrea Petkovic against No. 7 Vera Zvonareva; and No. 31 Kaia Kanepi against No. 15 Yanina Wickmayer, who made last year's semifinals.

On Sunday, No. 3 Venus Williams, No. 6 Francesca Schiavone, No. 2 Kim Clijsters and No. 5 Sam Stosur earned their spots in the quarterfinals. Stosur's 6-3, 2-6, 7-6 (2) win over No. 12 Elena Dementieva ended at 1:35 a.m., the latest finish for a women's match in US Open history. Her next opponent is Clijsters, who wrapped up her 6-2, 6-1 win over Ana Ivanovic shortly after the lunch hour Sunday.

"She's got a 14-hour head start on me," Stosur said. "She played first and I played last, so there's not much bigger difference than that."

Stosur saved four match points in a thrilling third set as she fought back to beat Dementieva.

The fifth-seeded Australian won the climactic tiebreaker 7-2 to end the titanic, two-hour 38-minute encounter at 1:35 a.m. (0535 GMT) in the latest-ending women's match ever staged at the US Open.

"That's definitely one of the most exciting matches I've ever played," Stosur told reporters. "The atmosphere out there was just awesome. I think we both played a great match."

The third set produced the best tennis of the women's tournament so far, with French Open finalist Stosur saving one match point when 12th seed Dementieva served at 5-3 and three more on her own serve in the next game.

The see-saw struggle saw both players raise their games in the final set, ripping groundstrokes with abandon and going for the lines.

In the end it was Stosur who held her nerve best, drawing errors from the usually steady Russian and clinching the decider on two forehand errors from the Olympic champion, who was runner-up at the US Open in 2004.

The women's match ended the night program on center court, following a men's third-round match won by 12th-seeded Russian Mikhail Youzhny over American John Isner in four sets.

"It's tough," Dementieva said. "We were waiting for a long time before we went on the court.

"It was probably one of the latest matches I ever played in my life. It was really difficult to stay focused. We had ups and downs all the time, both of us, during the match."

Stosur seemed to fall out of synch in the second set, losing her serve all four times and spraying unforced errors.

The Australian lost serve again in the second game of the third set to concede her fifth successive break and fall behind 3-0. She got back on serve with a break in the fifth game but Dementieva broke again to take a 5-3 lead.

That is when Stosur produced her most courageous tennis.

"I think the main thing is you want to play a tough point," said Stosur, who also saved four match points in winning a match last week in New Haven, and who saved a match point to beat Serena Williams at the French Open.

"If you get the opportunity to step in and go for a winner, of course do it. But you just want to make them earn it."

Stosur had never before got past the second round at Flushing Meadows.

"I think the last year and half I've been playing much better and really found my confidence and belief," she said. "So to come here and get my best result ever is great."

"I think she's won 18 matches in a row here," Stosur said of the Belgian, who also won in 2005 before her temporary retirement. "It's not going to be easy, but I'll give it my best."