Dominika Cibulkova, the No. 20 seed, took out Sharapova 3-6, 6-4, 6-1. After Sharapova used three breaks to win the first set, Cibulkova jumped out to a 5-0 lead in the second, was in control throughout the third set and claimed her first quarterfinal berth at the Open.
Sharapova, who entered the tournament as the No. 3 seed, hampered her own efforts with eight double faults among 45 unforced errors -- 34 of which came in the last two sets.
No. 11-seeded Simona Halep gained her first quarterfinal in any Grand Slam event with a 6-4, 2-6, 6-0 decision over Jankovic, the No. 8 seed. After Jankovic evened the match with a three-break second set, Halep opened the third set by taking a long game, which seemed to spur her to finish strong. She won 31 of the 45 points in the set.
Azarenka allowed Stephens just two break chances and saved them both. She won nearly half (32-of-65) of the points off Stephens' serve and was 3-of-14 in break opportunities.
The fourth round was completed with Radwanska's win over Muguruza. The match got off to a sluggish start but once Radwanska notched her first break -- she finished with four -- she moved quickly to the win. Muguruza, who was 0-for-6 in break chances, had 44 unforced errors to account for more than half of the 76 points Radwanska won.
With No. 1-ranked Serena Williams, Sharapova and Jankovic falling in the past two days, only three of the women's top eight seeds have reached the quarterfinals in Melbourne.
No. 4-seeded Li Na takes on 28th-seed Flavia Pennetta and No. 14 Ana Ivanovic plays 30th-seeded Eugenie Bouchard in the top of the draw. Halep meets Cibulkova and Azarenka goes against Radwanska in the other quarterfinal pairings.