Venus Williams came through a stiff test against Serbian schoolgirl Ana Ivanovic on Thursday to book her place in the Zurich Challenge quarter-finals.

The American former world number one eventually defeated the 16-year-old 7-6, 7-6 but will have to raise her game against in-form Wimbledon champion Maria Sharapova in the next round.

"There were definitely times when I thought to myself 'this shouldn't be happening'," Williams said. "I was always in control, but there were a lot of times when I let her back in the game when I shouldn't have done."

Williams broke early in the first set and seemed on course for a routine victory but world number 156 Ivanovic thrilled the Swiss crowd by storming back to force a tiebreak.

Ivanovic had five set points before Williams clinched the tiebreak 13-11.

The young Serb, who has won all five of the ITF tournaments in which she has competed this season, maintained her form in the second set to cause the four-times grand slam champion just as much trouble.

After trading service breaks Ivanovic again found herself with three set points, wasting one golden opportunity when she netted an attempted winner with the court at her mercy.

Williams finally sealed victory in one hour 48 minutes.

DEMENTIEVA THROUGH

Earlier third seed Elena Dementieva came through a scrappy battle with Russian Fed Cup team mate Elena Bovina 7-5, 3-6, 6-0.

Both players dropped serve five times in the opening two sets before Dementieva finally found her rhythm to romp past Bovina in the deciding set.

The French and U.S. Open runner-up has not enjoyed the best of luck against Russian players this year, losing to Anastasia Myskina and Svetlana Kuznetsova in both grand slam finals.

She has also been beaten by Russians in three of her last five tournaments and another showdown could be on the cards here where she may face Sharapova.

For that to happen Dementieva must overcome Japanese ninth seed Ai Sugiyama and Sharapova must beat Williams.

"I knew of course that there would be a lot of all-Russian matches this year, because all of us are playing so well," Dementieva said.

"But for me, it's not about the nationality of my opponent. I've always said I want to be number one in the world, not number one in Russia -- so if I need to beat other Russians to reach that goal, that's what I'll do."

Switzerland's Patty Schnyder maintained home interest, cruising past Colombia's Fabiola Zuluaga 6-4, 6-3.

The 10th seed will now meet Argentina's Paola Suarez in the last eight.