Chan Yung-Jan beat new British No.1 Heather Watson 6-3 7-6 in the second round at the WTA hardcourt event in Carlsbad.
Taiwanese qualifier Chan, ranked 161st in the world, 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, where 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 so it was difficult physically."
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 the flight to London next week, never faced a break point in a victory that took less than 90 minutes.
Top seed Marion Bartoli was forced to dig deep before over coming Vania King 6-4 5-7 6-4
