French Open champion Maria Sharapova admitted she had to dig deep to reach the quarter-finals of Stuttgart’s WTA tournament in a three-hour marathon victory over claycourt specialist Lucie Safarova on Thursday.

Sharapova, who won last year’s Stuttgart title en route to her Roland Garros victory, needed three hours, nine minutes to squeeze past her Czech opponent for a 6-4 6-7(3-7) 6-3 second-round victory.

With a 5-2 lead in the third set and serving for the match, Sharapova squandered three match points as Safarova broke her, before the Russian returned the compliment to claim the match.

“I’ve had a few three-hour games in my career, so I knew I just had to keep fighting until the end,” said the statuesque Russian who faces former world number one Ana Ivanovic in Friday’s quarter-finals.

“Clay is one of her favourite surfaces and I knew this was going to be a tough game, so I am glad to get through.

“The first match of the claycourt season is always tough, it’s nothing like practising.”

Sharapova’s last eight opponent Ivanovic beat Russia’s Nadia Petrova 6-4 6-3 in her second round game, having dropped to 17th in the world rankings.

Second-seed Li Na will face Petra Kvitova in the quarter-finals.

Li, who reached January’s Australian Open final, breezed through Thursday’s second-round match in 74 minutes to see off Croatia’s Mirjana Lucic-Baroni 6-1 6-2 in straight sets in her first match of the claycourt season.

Kvitova made the last eight after beating Germany’s Julia Goerges 2-6 7-6(7-4) 6-2.

Third-seed Angelique Kerber, ranked sixth in the world, also had no problems as she blasted past Russia’s Anastasia Pavlyuchenkova 6-0 6-4 to set up a quarter-final clash with Kazakhstan’s Yaroslava Shvedova.

Former world number one Jelena Jankovic of Serbia crashed out in the second round when she was beaten in straight sets 7-6(7/3), 7-5 by Germany’s Sabine Lisicki.