Second-seeded Agnieszka Radwanska set up a quarter-final clash with former US Open champion Samanth Stosur at the WTA Southern California Open Thursday with a straight-sets win over Daniela Hantuchova.
Poland's Radwanska, the 2010 runner-up and 2011 champion on the hardcourts of Carlsbad, defeated Hantuchova 6-3, 6-3, her sixth victory over the Slovakian in eight career meetings.
She was joined in the quarter-finals of the $795,707 US Open tuneup event by younger sister Urszula, a 6-2, 4-6, 6-3 winner over sixth-seeded Serbian Jelena Jankovic.
Urszula Radwanska's victory was just one of the upsets of the day, with French wild card Virginie Razzano ousting eighth-seeded Carla Suarez Navarro of Spain 6-1, 6-4.
Razzano next faces third-seeded Czech Petra Kvitova, a former Wimbledon champion.
From 2-2 in the third set, Radwanska won four of the next five games from former world number one Jankovic as she booked her fourth WTA quarter-final appearance of the season.
"I'm very happy that I won this match today," she said after earning a shot at top-seeded Victoria Azarenka of Belarus.
"This is a huge step for me because she was No.1 in the past, and I was playing her for the first time.
"I knew I had to fight for every point and be focused. I think the key is when I was up 3-2 in the third set and then I won two games pretty easily."
Radwanska talked about how her older sister's success has motivated her.
"When I see she's winning, I want to do the same," she said. "I want to be as good as she is. She's been at the top level for a few years. I'm still getting there. On the court, we are a little bit different. She's a little bit quiet. I'm more emotional. I hope one day I'll be in the spot where she is now."
Another former world number one from Serbia, Ana Ivanovic, made it safely through to the quarters, defeating US qualifier Coco Vandeweghe 7-6 (7/5), 6-3.