Top seed Caroline Wozniacki was stunned 6-2 7-5 by unseeded Israeli Shahar Peer in the third round of the Dubai tennis championships on Wednesday.
Second seed Svetlana Kuznetsova was also beaten, falling 5-7 7-6 6-4 to Russian qualifier Regina Kulikova.
Defending champion and third seed Venus Williams was tested in the early stages by Olga Govortsova of Belarus, falling behind 4-1, but she rallied to claim the final 10 games to win 7-5 6-0.
Fourth seed Victoria Azarenka also struggled before beating German qualifier Anna-Lena Groenefeld 6-1 4-6 6-3.
Wozniacki faced break points in every one of her 10 service games and she was broken eight times as Peer, ranked 22nd in the world, dictated events with her more aggressive play.
The Danish world number three failed to hold serve at all in the first set and managed just two holds in the second, for 1-0 and 3-2.
Despite her dominance, Peer struggled to close out the win. She served for the match twice, after breaking to lead 5-4 with a smash and then again at 6-5 when she had broken with a fine crosscourt winner.
A double-fault gave Wozniacki a break point to take the set into a tiebreak but she netted her return on the next point and Peer finally ended the match on her fourth match point.
"She played well today, she really did," Wozniacki told reporters. "She was changing the rhythm and going in and attacking the balls. I had my chances, especially in the second set, and I didn't take them."
Russian world number four Kuznetsova struggled throughout her three-hour-16-minute battle with Kulikova, who this week has won back-to-back matches at a Tour event for only the second time.
The 21-year-old, ranked 99, built up a 4-1 lead, and although Kuznetsova sneaked the first set she allowed Kulikova to dominate the tiebreak in the second and win the final five games in the third.
Poland's Agnieszka Radwanska took just an hour to reach the quarter-finals, defeating 10th-seeded Italian Flavia Pennetta, who was struggling with a hip strain, 6-3 6-0.
