STUTTGART, Germany, Oct 2, 2008 (AFP) - Russia's Olympic stars Dinara Safina and Vera Zvonareva produced impressive performances on Thursday to book their places in the quarter-finals of Stuttgart's WTA event.

Beijing silver-medalist Safina had to work for her 6-1, 7-6 (8/6) win over Bulgaria's Tsvetana Pironkova, while Zvonareva, who won bronze, dominated France's Marion Bartoli for a 6-2, 6-0 victory.

Having needed just 21 minutes to take the first set 6-1, Safina was made to work hard in the second as the 21-year-old, 49 places below her in the WTA rankings, took the set to a tie-break with a clever selection of shots.

Pironkova was regularly hitting her serves over 190km/h and smashed four aces in the second set, but Safina put her under pressure in the tie break and the Bulgarian's inexperience played into her opponents' hands.

'She's a good player, but I had plenty of opportunities to win the match,' said Safina who will play either Venus Williams or Kateryna Bondarenko in the quarter-finals.

'I became a little bit slow in the second after I dominated the first.

'I lost my concentration, but it was good to have a hard second set - it will help me for later in the week.'

Zvonareva made few mistakes and needed just 58 minutes for her straight sets win over Bartoli and will play either world number two Jelena Jankovic or Alona Bondarenko on Friday for a place in the semi-finals.

And 2006 winner Nadia Petrova is safely in the quarter-finals after her second round 6-4, 6-1 win over Switzerland's Patty Schnyder.

Petrova will play China's Li Na, who beat world number one Serena Williams on Wednesday, in the quarter-finals.