Top seed Maria Sharapova had to battle for more than three hours to overcome Anabel Medina Garrigues at the Amelia Island Championships.
The Russian booked her quarter-final place with with a 7-6 5-7 7-6 win.
"In those situations, it all comes down to the heart and how tough you are," she said. "The forehands or backhands don't mean much after three hours."
Sharapova joined Amelie Mauresmo and Lindsay Davenport in the last eight but second seed Anna Chakvetadze is out.
Russian Chakvetadze lost 6-2 3-6 6-1 to Slovakia's Dominika Cibulkova - who will go on to face Mauresmo.
Former world number one Mauresmo lost the first set against Polish teenager Agnieszka Radwanska before coming through 3-6 7-5 7-6.
Meanwhile, Davenport, seeded 16th, defeated Karolina Sprem 6-4 3-6 6-4.
Sharapova felt she only had herself to blame for letting Medina Garrigues back into the match after winning the first set.
"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, but I stopped hitting the ball," she said.
Sharapova forged a a 5-3 lead in the final set but was rattled by a controversial line call in the ninth game and had to serve to stay in the match at 5-6.
However, she dominated that game and then went on to sweep through the tie-break 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'."
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.