STUTTGART, Germany, Oct 3, 2008 (AFP) - Olympic champion Elena Dementieva exited Stuttgart's WTA event after a marathon quarter-final on Friday as Bulgarian teenager Victoria Azarenka booked her semi-final berth here.

The 19-year-old, who also reached the semi-finals of the Berlin Open in May, came out on top after nearly three hours of tennis to knock out the fourth seed with a 7-6 (8/6), 3-6, 6-1 victory.

'I just kept fighting,' said Azarenka as her victory came after two hours 53 minutes.

'She is such a good opponent, I had to fight for every ball, but my legs are pretty dead now.'

The first set alone lasted 62 minutes as Azarenka held her nerve to win the tie-break, but the second set was also a dogged affair which lasted 67 minutes as Beijing gold medalist Dementieva rallied for a 6-3 win.

Fatigue hit both players as the game moved into its third hour, but Azarenka was the stronger as she bolted into a 5-0 lead before taking the third set 6-1.

This is the second time this season Azarenka, ranked 17th in the world, has reached the last four of a tournament in Germany having lost the semi-final to Dinara Safina in Berlin when she played despite carrying a leg injury.

'I think I have to come to Germany more often,' she said.

Azarenka will play either China's Na Li or Russia's Nadia Petrova - who won the event in 2006 - in Saturday's semi-final.