Top seed Marion Bartoli survived a struggle lasting for more than three hours, with the Frenchwoman overcoming a massive 18 double-faults to advanced at the WTA hardcourt event.
Bartoli defeated American Vania King 6-4, 5-7, 6-4 in an error-plagued marathon match to reach the quarter-finals after a bye in the first round.
"It's a very hard surface," Bartoli said. "Playing three hours is not really helping, but I think it was a great fight from both of us and we tried our best."
Bartoli, who will not play in the London Olympics due to a longstanding dispute with the French federation, won a battle in which both women took lengthy injury breaks, with Bartoli going off court for treatment in the third set.
Bartoli said she felt her hip hurting since the first set.
"I really tried to keep the pain away and tried to not think about it," she said.
"Obviously it came to a point where I had really a lot of problems serving. I really needed some help from the trainer.
"It was a gradual pain. At the end of the first set I started to feel it but was able to go through it. And then in the second set we had some tough rallies, and the more she was moving side to side, the more I was feeling it."
Bartoli advanced despite losing serve eight times against the world 59th-ranked King.
The French player lost serve in the opening game of the final set but fought to get it back as she broke for four-all before closing out the match at La Costa on a concluding break.
US Olympian Varvara Lepchenko upset South African eighth seed Chanelle Scheepers 6-4, 6-2 to reach her third quarter-final of the season.
The 44th-ranked Lepchenko, who will join Venus and Serena Williams plus fellow Carlsbad competitor Christina McHale on a flight to London next week for the July 28 start of the Olympic tennis event, never faced a break point in a victory that took less than 90 minutes.
Lepchenko reached quarter-finals this season indoors in Memphis in February and on the blue clay of May's Madrid Masters, where she lost to Agnieszka Radwanska.
The American broke Scheepers three times and advanced as the South African sailed a return long.
"I'm still improving and still trying to prove to myself out there that I can play on all the surfaces," said Lepchenko. "A lot of people put me into a certain category. But I fell I can play on all surfaces, and I've been showing it."
Lepchenko next takes on fourth seed Nadia Petrova, who beat American Alexa Glatch 6-4, 6-3.
Petrova's tournament participation dates back to 2001, with her best showings being a pair of quarter-finals in 2003 and 2007.
The 30-year-old Russian with 11 WTA titles, who won her last event on grass in the Netherlands the week prior to Wimbledon, is Olympic-bound next week.
Taiwanese qualifier Chan Yung-Jan beat Britain's Heather Watson 6-3, 7-6 (7/2).
Chan, ranked 161st, will face a Friday test against either fourth seed Jelena Jankovic, the former world number one Serb, or Hungarian Melinda Czink.
Chan won the opening set in 40 minutes, breaking Watson in the final game as the British player, who failed to convert on any of her seven break points, served to stay in it.
In the second, six service breaks came thick and fast as the set went to a tiebreaker. Chan took a 5-2 lead and put away a crosscourt volley to earn the victory.
"Today was a difficult match because both of us are pretty solid," Chan said. "The weather was pretty warm today, so it was difficult physically."