Samantha Stosur will face surprise packet Sesil Karatantcheva for a place in the quarter-finals at the Southern California Open after the Kazakh qualifier upset Julia Goerges in the opening round.
Karatantcheva recovered from an early service break down to beat Goerges 6-3 7-5 and advance to a second-round meeting with fifth-seeded Stosur.
Slovakian Daniela Hantuchova roared into the second round with a 6-0 6-0 thrashing of Austrian Tamira Paszek.
Hantuchova, who set up a second-round clash with second-seeded Agnieszka Radwanska, also warmed up for her marquee doubles match on Wednesday with former world No.1 Martina Hingis.
Swiss great Hingis will be playing her first WTA match since 2007 when she and Hantuchova play first-round doubles against Goerges of Germany and Croatian Darija Jurak.
Hingis won five grand slam singles titles and nine in doubles and spent a total of 209 weeks as world No.1 before she retired for the first time in 2003.
A comeback bid ended in a second retirement in 2007.
Top seed Victoria Azarenka will also be in action on Wednesday, opening her campaign with a second-round match against 2010 French Open winner Francesca Schiavone of Italy.
The only seeded player in action on Tuesday was eighth seed Carla Suarez Navarro, who eased into the second round with a 6-4 6-3 victory over Japanese qualifier Sachie Ishizu.
In other matches, Urszula Radwanska rallied from a break down four times in the opening set on her way to a 7-6 (7-3) 6-4 victory over New Zealand qualifier Marina Erakovic.
Britain's Laura Robson also had to battle. Japan's Ayumi Morita served for their match at 6-4 5-4, taking a 30-0 lead, but couldn't close it out and eventually fell 4-6 7-5 6-2.
US qualifier Coco Vandeweghe sped into the second round, downing wildcard Allie Kiick 6-1 6-2.
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