Keothavong crashes out in Stanford
Anne Keothavong failed to follow the lead of Heather Watson - who was a first-round winner - as she crashed out of the Bank of the West Classic to US wildcard Mallory Burdette.
The British No. 1 was expected to prove too good for her unranked opponent, but she surrendered a position of dominance to fall to 2-6 7-5 6-4 defeat.
Things started well as Keothavong broke on four occasions to take the opening set but her opponent dug in to take a tight second set.
Burdette could have been forgiven for feeling somewhat tense in the final set, given she had only won one previous match as a professional, but she held her nerve to claim the win.
Burdette's reward is a clash with second seed Marion Bartoli.
Teenager American Nicole Gibbs claimed a 6-4 6-4 win over Noppawan Lertcheewakarn and her reward is the biggest game of her career: a clash with Wimbledon champion Serena Williams.
"I'm so in awe of her. She was one of my idols growing up. I'm just going to be so honoured to be on the court with her tomorrow," Gibbs said. "I was her ballgirl once when I was 12 years old. I spoke to her briefly in the locker room there. We got to hit with them for five minutes and it was the best memory of my life.
"For me it's already a dream to be on the court with someone that great."