Caroline Wozniacki battled through two tie-breaks to overcome Barbora Zahlavova Strycova 7-6 (8-6) 7-6 (11-9) at the Family Circle Cup in Charleston.
The world number one was on court for two-and-a-half hours against the Czech world number 48 before she closed out the tie to reach the quarter-finals.
Defending champion Sam Stosur fell to a shock 6-4 6-1 defeat by Elena Vesnina.
Second seed Stosur, the French Open runner-up in 2010, was comfortably beaten by the Russian world number 56.
On a disappointing day for the tournament's top seeds Nadia Petrova was undone by China's Shuai Peng 6-3 5-7 6-2. The Russian seventh seed seemed to have worked her way back into the match after losing the opening set but was easily undone by the 11th seed in the third.
Daniela Hantuchova, seeded 10th, was beaten 7-6 (7-3) 6-1 by American Christina McHale.
And German 12th seed Julia Goerges knocked out Israel's world number 11 Shahar Peer, who was ranked fifth at Charleston, 6-2 6-3.
Elsewhere, Serbian third seed Jelena Jankovic eased to a 6-2 6-2 victory over South Africa's Chanelle Scheepers.
Belgian sixth seed Yanina Wickmayer beat Anna Tatishvili of Georgia 6-4 7-5 and goes on to face Wozniacki next.
It proved to be a tough workout for top seed Wozniacki, who had won her opening match in convincing fashion, dispatching American qualifier Irina Falconi 6-1, 6-1.
The Dane led 4-1 in the opening set but Zahlavova Strycova fought back only to miss a set point in the tie-break.
"I knew that it wasn't going to be easy. I've seen her before and played her last year here," said Wozniacki, who missed four match points in the second tie-break before finally seeing off her opponent.
"This is the first tournament on clay. You need to get used to the conditions, you need to get used to sliding.
"There will be longer rallies in clay, there's no doubt about it, and the matches will most likely be longer."
Meanwhile, at the Andalucia Tennis Experience in Marbella, top seed Victoria Azarenka continued her fine run of form. The world number six from Belarus, who triumphed at the Sony Ericsson Open last week, clinched a 6-3 6-2 win over world number 82 Laura Pous-Tio.
Fourth seed Alexandra Dulgheru fought back from losing the first set to overcome Anabel Medina Garrigues 5-7 6-4 6-4 in a contest that lasted two hours and 57 minutes.