Wozniacki is currently ranked third in the world
Top two seeds Caroline Wozniacki and Svetlana Kuznetsova were both sent crashing on a day of upsets in the third round of the Dubai Championships.
Number one seed Wozniacki, runner up at last year's US Open, was beaten in straight sets by Israeli Shahar Peer.
Peer had never before taken a set off the Dane in three previous matches but eased her way to a 6-2 7-5 triumph.
And Kuznetsova suffered an even bigger shock as she lost to Russian qualifier Regina Kulikova 7-5 6-7 (2-7) 4-6.
Kulikova had never met two-time Grand Slam winner Kuznetsova, but reeled off the last five games in the final set to win from 4-1 down.
"I don't know how I won. I was just playing," said Kulikova. "She is one of the best players in the world."
Peer, meanwhile, was delighted after reaching at least the quarter-finals for the fourth time in five tournaments this year.
"I played very, very good, and I'm really happy," stated Peer, who was refused a visa to play in the tournament last year.
Tournament organisers were subsequently fined $300,000 after denying Peer a visa on the grounds of security concerns.
Defending champion Venus Williams, meanwhile, progressed with a 7-5 6-0 victory over Olga Govortsova of Belarus.
The American made a poor start and trailed 4-1 in the first set but recovered to win it and dominated the second.
Fourth seed Victoria Azarenka dropped a set against qualifier Anna-Lena Groenefeld but came through 6-1 4-6 6-3 in the end while Agnieszka Radwanska, seeded seventh, saw off Flavia Pennetta 6-3 6-0.