The defending champion fell to French ninth seed Marion Bartoli, who took the first set with a surprising 6-3. The American recovered well in the second set, breaking serve to take it to a tense tie-break that Bartoli stole 8-6.
Bartoli, who lost the final of 2007’s tournament to Venus Williams, will face Sabine Lisicki of Germany in the quarter final.
Meanwhile, Andy Murray sealed his place in the final eight in his straight-sets victory against Richard Gasquet.
The match appeared to be anyone’s in the first set, which ended in a 7-3 tie-break, but the Frenchman faltered in the second set, ending 6-3. Fourth seed Murray sustained the momentum to win 7-6 (7-3) 6-3 6-2.
He will next face either Spain’s Feliciano Lopez or Pole Lukasz Kubot, both unseeded.