Serena Williams booked a marquee quarterfinal against Maria Sharapova on Thursday as top seed and defending champion Victoria Azarenka was sent packing at the $721,000 WTA Stanford tournament.
New Zealand qualifier Marina Erakovic, ranked 121st in the world, stunned Azarenka 4-6, 7-5, 6-2. She notched her first victory over a top-20 opponent and reached her second quarter-final of a season in which she has won 24 of her last 28 matches.
Belarus' Azarenka, ranked fourth in the world, arrived in Stanford for this hardcourt US Open tuneup off a semi-final appearance at Wimbledon.
She may have been a little rusty after the first-round bye given to the top four seeds.
Williams had no such luxury, despite her 13 Grand Slam titles. The US star's ranking has plunged in the wake of her 11-month layoff due to illness and injury and she is unseeded in this event.
Playing her third WTA event since her return, Williams lined up a clash with second-seeded Sharapova with a 6-2, 3-6, 6-2 victory over Russian Maria Kirilenko.
Erakovic next faces eighth-seeded Slovakian Dominika Cibulkova.
"Coming in I knew I had to be the aggressor and try to dictate the pace," Erakovic said. "I knew the court and I felt like I was hitting the ball well."
The 23-year-old Croatian-born Erakovic qualified for the main draw at Wimbledon and won her first-round match there. She also qualified for the French Open.
It's a welcome resurgence for a player who missed six months with a hip injury in 2009 and came back on the lower-level ITF circuit, where she has won a dozen titles.
"I went into a slump just before the injury and was beginning to question whether this tennis thing was for me," Erakovic admitted. "The injury made me realize how much I love tennis and that's when I committed myself. Coming back I feel like I can compete with the best."
In other early matches, Japan's Ayumui Morita booked a quarter-final berth against French third seed Marion Bartoli. Morita defeated qualifier Urszula Radwanska of Poland 4-6, 7-5, 6-1.
Fourth-seeded Australian Samantha Stosur was due to open her campaign with a second-round clash with Germany's Wimbledon semi-finalist Sabine Lisicki.