Keothavong overcame Sofia Arvidsson in the last round
Anne Keothavong bowed out of the Aegon Classic in the second round, but fellow Briton Naomi Cavaday produced a surprise win over Tamarine Tanasugarn.
British number one Keothavong, seeded 11th for the event, was beaten 6-1 7-6 (7-5) by India's Sania Mirza.
Cavaday made light of the 250 places separating her and Tanasugarn in the rankings to win 7-6 (7-5) 2-6 6-4.
She will now face Stefanie Voegele, who caused an upset of her own in beating Russia's Ekaterina Makarova.
After losing the opener, Tanasugarn, a Wimbledon quarter-finalist last year, seemed to have wrestled back control of the contest in the second set.
But Cavaday produced a break in the seventh game of the decider and clung on to her own serve to claim a memorable victory.
"The biggest thing today was my belief. I never stopped believing in myself and that's what got me through during the tough times in the match," said Cavaday.
"This is my best win in rankings terms as she is 47 in the world. I just love it here."
Mirza wrapped up the first set of her match against Keothavong in emphatic style as the Londoner managed to hold just one of her four service games.
The 25-year-old Briton recovered in the second set and seemed poised to level at 5-2.
There are plenty of things for me to work on before Eastbourne and Wimbledon
But Mirza successfully defended two set points which, combined with nine saved break points throughout the second, carried the match into a tie-break.
The unseeded 22-year-old, who has sunk to 98th in the world from a career-best of 27th two years ago, then held her nerve to set up a possible meeting with Britain's Elena Baltacha in the third round.
"I didn't expect to play my best tennis at this tournament because it's my first grass-court tournament," said Keothavong.
"You can't guarantee a good performance every day unfortunately and there are plenty of things for me to work on before Eastbourne and Wimbledon."
Switzerland's Voegele ended sixth seed Makarova's campaign with a 6-3 3-6 6-2 win in a match that lasted just over two hours.
But there were no such problems for number three seed Aleksandra Wozniak who progressed to the next stage with a 6-4 6-4 victory over Belarussian Olga Govortsova.