MELBOURNE, Australia -- 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.

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.

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.

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.