British number one Elena Baltacha went out of the Mercury Insurance Open in Carlsbad with a straight-sets defeat by Poland's Agnieszka Radwanska.
Baltacha, 27 was beaten 6-0 7-6 (7-4) by the world number 13 after battling her way back into the match.
The Scot recovered from dropping the first set in 26 minutes to lead the second 5-3, but could not convert four set points and Radwanska prevailed.
Their match was the only second-round tie to be completed on Tuesday.
In the first round, three seeds fell with number 10 Flavia Pennetta squandering a lead to fall 1-6 6-2 6-2 to fellow Italian Alberta Brianti.
Thirteenth seed Jarmila Gajdosova rarely put up a fight in going down 6-3 6-2 to Russian Alla Kudryavtseva, while number 14 seed Elena Vesnina surrendered 6-1 6-2 against qualifier Marina Erakovic.
The three other seeds in action all progressed with number 11 Maria Kirilenko topping Rebecca Marino 6-3 6-2, 12th seed Sabine Lisicki breezing past Greta Arn 6-0 6-2 and number 15 Sara Errani seeing off Sania Mirza 6-2 6-3.
Wildcard Sloane Stephens made it to the second round with a 6-4 6-2 win over Shuai Zhang, while Tamira Paszek rallied to beat Simona Halep 0-6 6-4 7-5 while Barbora Zahlavova Strycova was a 6-4 1-6 7-5 winner over Chanelle Scheepers.
Kimiko Date-Krumm beat Gisela Dulko 7-5 6-0, Anastasia Rodionova battled past Olga Govortsova 7-6 (7-4) 2-6 6-3 and Ayumi Morita beat Jelena Dokic 6-1 4-6 7-5.