California - Sixth-seeded Daniela Hantuchova of Slovakia lost 6-4, 7-5 Poland's Ursula Radwanska in the first round of the Mercury Insurance Open on Monday.
Hantuchova made an unforced error when she sent a forehand long to give the match to Radwanska.
Hantuchova, who has had an inconsistent season marked by a left foot injury that caused her to miss more than a month, dropped the first set when Radwanska registered the only service break in the fifth game.
Radwanska, the younger sister of world No 2 Anieszka Radwanska, traded service breaks with Hantuchova early in the second set. Radwanska broke serve again for a 5-4 lead before winning the match on her next service game.
Earlier, Varvara Lepchenko cruised to a 6-3, 6-1 win over fellow-American Nicole Gibbs.
Uzbekistan-born Lepchenko, a naturalized US citizen and member of the US Olympic team for London, used a variety of shots including numerous drop shots for winners as she advanced to the second round.