DUBAI, United Arab Emirates — Top-seeded Serena Williams and her sister Venus moved closer to a semifinal matchup at the Dubai Tennis Championships with third-round victories Wednesday.

Serena Williams defeated 13th-seeded Zheng Jie of China 6-4, 6-2, with sixth-seeded Venus Williams topping 11th-seeded Alize Cornet of France 6-3, 6-2.

Third-seeded Jelena Jankovic of Serbia was upset by emerging talent Kaia Kanepi of Estonia 6-2, 7-5.

The WTA event attracted nine of the top-10 players, but the tournament is being overshadowed by the Emirates' refusal to grant a visa to Israeli player Shahar Peer, a move that could lead to the event being struck from next year's calendar.

Also advancing to the quarterfinals were eighth-seeded Ana Ivanovic of Serbia and defending champion Elena Dementieva of Russia. Ivanovic beat Camille Pin of France 6-2, 7-6 (5) and Dementieva defeated Anabel Medina Garrigues of Spain 6-1, 6-3.

Kanepi dominated Jankovic with powerful strokes and serves. Jankovic got her first break in the fifth game of the second set when the 23-year-old Estonian double faulted on second break point.

Down 6-2, 4-0 at one stage, Jankovic pulled even in the second set at 5-all before Kanepi sealed the match with a break in the 12th game.

Jankovic, a former No. 1, called the loss the "worst match of my career.

"It was a horrible day," she said. "I kept framing the ball, kept making unforced errors and could not put two balls together on the court. I didn't move properly, and I didn't see the ball properly. She didn't have to do too much. Basically, I beat myself. I don't know what happened out there. I am ashamed of this performance."

Kanepi, whose previous biggest victory came against sixth-ranked Anna Chakvedatze in last year's French Open, said: "I am really delighted. It was my tactic to mix it up and it worked."

Also, fifth-seeded Vera Zvonareva of Russia advanced when former Wimbledon finalist Marion Bartoli retired while trailing 2-5 because of foot blisters. Dominika Cibulkova of Slovakia also retired after feeling sick in the deciding set against Russian qualifier Elena Vesnina.