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.

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.

Sharapova lost 3-6, 6-4, 6-1 after making 45 unforced errors.