Dementieva cruised to a 6-3, 6-1 victory for her first win over a top-10 player since defeating the then-No. 1 Hingis in the quarterfinals at Moscow in 2001. The 20-year-old Dementieva, a runner-up at the Ordina Open in June, will take on unseeded qualifier Myriam Casanova of Switzerland in the quarterfinals.
"I'm very happy with this match, actually my last two matches," Dementieva said. "I know the score looks easy, but it was a tough match. I have a lot of respect for Martina and it can't be easy for her coming back from her injury. She still has the best tactics and hands on the tour, I just don't think she's moving that well at the moment."
The loss will drop Hingis out of the top 10 rankings for the first time since the week of Sept. 30, 1996. Hingis, who underwent surgery to repair torn ligaments in her ankle last October, has two titles this season. But she has failed to reach the semifinals in her last five starts.
"I played well for the first six games or so," Hingis said. "My problem was that I wanted to make the game too quickly. In the first set, I spent a lot of energy and in the second set I got a little frustrated. It was a kind of blackout. Physically I'm feeling better, but we all have our ups and downs."
Casanova also posted an upset Thursday, recording a 7-5, 4-6, 6-4 victory over fourth-seeded Justine Henin of Belgium. It was Casanova's first win over a top-5 player.
"That was the fight of my life," Casanova said. "After this match, first I felt like I was paralyzed. Then I cried a little while and now I am beginning to understand what I did. It is an incredible feeling."
Russian Tatiana Panova overcame No. 3 Jelena Dokic of Ygoslavia, 1-6, 6-4, 6-1, marking the second time ever that the third and fourth seeds lost their opening matches here.
Fifth seed Amelie Mauresmo of France and sixth-seeded Belgian Kim Clijsters avoided upsets Thursday. Mauresmo routed Luxembourg's Anne Kremer, 6-1, 6-2 and Clijsters beat Croatian Iva Majoli by the same score.
On Wednesday qualifier Alexandra Stevenson ousted top-seeded Jennifer Capriati, 7-6 (7-4), 4-6, 6-4.