Two-time defending champion Victoria Azarenka saw another of her main adversaries crash out of the Australian Open on Monday while top-ranked Rafael Nadal and Wimbledon champion Andy Murray faced tougher-than-expected matches to advance to the quarterfinals at Melbourne Park.

Maria Sharapova joined Serena Williams as fourth-round casualties, leaving Azarenka a potentially easier run to the women's final. Sharapova was beaten by No. 20 Dominika Cibulkova in the match on Rod Laver Arena immediately before Azarenka defeated Sloane Stephens, the last remaining American in the singles draw.

Nadal won three tough sets against Kei Nishikori, while Murray wasted four match points in the third set — including two in the tiebreaker — before dominating the fourth set against lucky loser Stephane Robert of France.

Four-time Australian champion Roger Federer was playing 2008 finalist Jo-Wilfried Tsonga in a late match, with the winner moving into a quarterfinal match against Murray.

Third-seeded Sharapova, complaining of a hip ailment, lost 3-6, 6-4, 6-1 after struggling with her serve and made 45 unforced errors. She joins Serena Williams on the sidelines after the top-ranked American lost in three sets Sunday to Ana Ivanovic, ending 25-match winning streak.

Sharapova spent the latter months of last year on the sidelines with a right shoulder injury, so wasn't expecting much of herself.

"I have to look at the positives and see where I have come from in four or five months," Sharapova said. "I haven't played a lot of tennis. I certainly would have loved to play a little bit more before playing a Grand Slam, but this is the chance that I was given."

Azarenka had a 6-3, 6-2 win over No. 13-seeded Stephens, the player who beat Williams in the quarterfinals here last year and then lost to Azarenka in the semifinals under controversial circumstances. Azarenka took a medical time out when Stephens started to take momentum in that match, then came back to advance to the final and subsequently her second Australian Open trophy.

She wasn't getting carried away Monday with the apparent ease of the draw with Williams and Sharapova gone.

"The players who beat those players deserve all the credit because they've been better, so they are dangerous and they are competitive," Azarenka said.

Nadal beat Nishikori 7-6 (3), 7-5, 7-6 (3) and was broken twice in the third set — getting a time violation from chair umpire Evanthia Asderaki in one of them. The 13-time major winner tripped in the first set and tumbled to the court, then had to go to the locker room to change shoes.

"I never bring a second pair of shoes on court," said Nadal, smiling. "My lace broke."

He was more serious while commenting on the time violation.

"Normally I have a good self-control on court, and it's true that the moment was a very critical one," Nadal said. "I know the rules are there but timings are sometime a little tough. The (umpire) made the decision, so I respect her even if I was not very happy with that decision."

Three-time Australian Open finalist Murray ended the run of Robert, wasting match points in the third set and smashing his racket after the tiebreaker before advancing 6-1, 6-2, 7-6 (6), 6-2.

"He's a fun player to watch, but not a fun player to play," Murray said of Robert's assortment of drop shots and deft groundstrokes.

Nadal next plays Grigor Dimitrov, who reached the quarterfinals of a Grand Slam for the first time with a 6-3, 3-6, 6-2, 6-4 win over Roberto Bautista Agut.

Dimitrov will continue on to the quarterfinals without his girlfriend, Sharapova, who was broken four consecutive times and fell behind 5-0 in the second set.

She reeled off four games to get back in contention before Cibulkova held to level the match.

After taking an off-court medical time out after the second set for her hip problems, Sharapova was broken immediately and had seven double-faults in the third set.

Azarenka, who is on an 18-match winning streak at Melbourne Park, will play either No. 5 Agnieszka Radwanska or Spain's Garbine Muguruza.

"I just love playing here, the surroundings, it feels so cozy ... feels like home," she said after her win over Stephens.

On Tuesday, the first of the quarterfinals begin when Ivanovic plays Eugenie Bouchard and two-time Australian Open finalist Li Na takes on Flavia Pennetta.

In men's quarterfinals, three-time defending champion Novak Djokovic plays Stanislas Wawrinka and No. 3 David Ferrer takes on No. 7 Tomas Berdych.