Top seed Maria Sharapova needed more than three hours to beat Anabel Medina Garrigues and reach the Amelia Island Championships quarter-finals on Thursday.

Sharapova took a staggering three hours, 27 minutes to overcome Medina Garrigues 7-6 5-7 7-6 as both players slugged it out from the baseline.

Australian Open champion Sharapova joins fellow grand slam winner Amelie Mauresmo in the quarter-finals after the 11th-seeded Frenchwoman survived a first-set wobble to defeat Agnieszka Radwanska 3-6 7-5 7-6.

"There were a lot of ups and downs in the match. I think I should have won the match in two sets to be honest," Sharapova told reporters. "But I stopped hitting the ball and I kind of let her back in the match."

Sharapova, who had held a 5-3 lead in the final set but was rattled by a controversial line call in the ninth game, was struggling at 5-6 before she dominated her service game to force a tiebreak.

The Russian had no problems in the tiebreak, running through it 7-1.

"When you're down 5-6 after being 5-3 up, you're just trying to think of the right things and just focusing on what you have to do in order to win point by point," Sharapova said. "I played a really good game to get it even, and I just thought 'You have to do the same thing in the tiebreaker'.

"In those situations, after three hours or so, it all comes down to the heart and how tough you are. The forehands or backhands don't mean much after that."

Sharapova will meet 10th seed Alona Bondarenko, who also advanced with a three-set victory over sixth seed Dinara Safina, 7-6 0-6 7-5.

Mauresmo will face Slovakia's Dominika Cibulkova, who upset second seed Anna Chakvetadze 6-2 3-6 6-1.

Cibulkova, who practised with Mauresmo last week in Miami, overcame early jitters and dominated the third set when she allowed her Russian opponent just 17 points.

"It probably wasn't my best tennis but she's a tough player," said Cibulkova. "I just tried to stay focus and play hard.

"This wasn't the first top-10 player I've beaten but beating her means a lot to me."

Former world number one Lindsay Davenport, seeded 16th, also reached the last eight with a 6-4 3-6 6-4 win over Croatian Karolina Sprem.