Stuttgart - Venus Williams overpowered Dinara Safina 6-4, 6-2 on Thursday to advance into the Porsche Grand Prix semi-finals, with her sights set firmly on a sports car waiting for the champion.

However, in order to stay in the hunt for the red Porsche 911 convertible she must beat new world number one Jelena Jankovic on Saturday in the semis.

"I love fast cars. I really hope to try it as the winner of the tournament," the Wimbledon champion Williams said.

The second seed Jankovic fought through the pain barrier of a broken toe nail to beat seventh seeded Russian Vera Zvonareva in the best match so far at the tournament, 7-6 (10-8), 7-6 (7-5).

Earlier, Belarussian teenager Victoria Azarenka upset Olympic champion Elena Dementieva of Russia 7-6 (8-6), 3-6, 6-1 in a 2:53-hour marathon.

The 17th-ranked Azarenka now plays another Russian in the form of 2006 champion Nadia Petrova, who ousted China's Li Na 6-2, 6-3.

The six seed Williams won seven games in a row from 4-3 down in the first set to 4-0 in the second en route to victory over the third seeded Russian Safina in their first meeting 1:16 hours. Williams served out the match at love, winning with her fifth ace.

Williams, 28, is enjoying a great week in her first appearance at the event in a decade.

She used her big serve, powerful groundstrokes and several excursions to the net to beat Safina, who also is a big-hitter and one of the hottest players of the year with four titles.

"I had to work really hard because she played at 200 per cent. It is exciting to win against such a great player who has had a great year," said Williams.

Jankovic and Zvonareva then engaged in an even bigger slugfest, with Zvonareva break up in each set but Jankovic came back each time to win in a tiebreaker.

She saved a set point at 7-6 in the opening set tiebreak and won on her third chance with a service winner. She wrapped up matters after two hours when Zvonareva netted a forehand after slipping.

Victory came after Jankovic had her left foot taped at 5-5 after damaging the nail when chasing a ball.

"I had no idea how to get through. I broke the nail and didn't know how to continue ... I fought until the end and really wanted to finish in two sets," said Jankovic, who reclaims the top ranking on Monday from Serena Williams.

Azarenka, 19, saved a setpoint in the first-set tiebreak and then fully dominated the final set. Dementieva saved three match points at 5-0 before hitting a backhand into the net on Azarenka's fourth chance in the next game.

"I just kept on fighting. We had many long rallies, I tried to fight for every ball. I was playing very good," said Azarenka.

Azarenka, who has one of the loudest grunts on the women's circuit, had lost the previous two matches against Dementieva in straight sets.

The 18th-ranked Petrova was in full command against world number 30 Li, who two days ago had upset American top seed Serena Williams. Petrova raised her perfect record against Li to 6-0 when she she clinched victory with an ace. (dpa)