Serena Williams will have to get past her sister Venus if she is to win a fourth successive Nasdaq-100 Open title after they set up a quarter-final showdown on Monday.
Venus needed three sets to get past feisty Colombian qualifier Catalina Castano 6-4, 2-6, 6-1, while Serena had to come from 3-1 down in the final set to beat Russian Elena Likhovtseva 6-4, 1-6, 6-4.
World number one Roger Federer also had a battle on his hands in his men's third round tie against Mariano Zabaleta before beating the Argentine 6-2, 5-7, 6-3 to notch up his 18th win in succession.
Tuesday night's quarter-final will mark the first meeting between the Williams sisters since Wimbledon in 2003.
"It's all business when we play each other, there's no kidding around out there," said Serena. "I wish it was later in the tournament but at least one of us is going to make the semi-finals."
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Play earlier in the day was again marked by upsets with French Open champion Anastasia Myskina falling victim to a resurgent Kim Clijsters in a 6-3, 6-4 defeat.
Myskina's compatriot and fellow grand slam champion Svetlana Kuznetsova also went out, the U.S. Open title holder losing 3-6, 6-3, 7-5 to 17-year-old Serbian Ana Ivanovic.
Clijsters' victory was her 11th in a row and she has now lost just once since returning to the tour last month after missing most of last season with a wrist injury.
The 21-year-old, who next faces fourth seed Elena Dementieva, was joined in the quarter-finals by her fellow Belgian Justine Henin-Hardenne, who is also on the comeback trail.
Playing her first tournament since last year's US Open following illness and injury, Henin beat seventh seeded Australian Alicia Molik 6-4, 2-6, 6-2.
Second seed Maria Sharapova had no intention of joining the list of seeded casualties and duly ran out a 6-1, 6-2 winner over Japan's Shinobu Asagoe.
SMOOTH PROGRESS
Top seed Amelie Mauresmo of France also made smooth progress, beating Croatia's Karolina Sprem 6-1, 7-5 to set up a quarter-final against Ivanovic.
Andre Agassi brushed aside pre-tournament worries about an injured toe and downed Frenchman Arnaud Clement 6-2, 6-4.
"I felt much more comfortable today than my first match, which is a sign that things have really moved forward physically, and also with my game," said the ninth seed. "I'm looking forward to getting into the thick of things here."
But Guillermo Coria, last year's runner-up, joined the roster of men's seeds ousted over the first few days of this year's event when he lost 6-3, 6-4 to American Taylor Dent.
"I didn't play my best tennis today and he played very well, so it was no surprise," said the fourth-seeded Argentine. "He was stronger mentally and came up with bigger shots at the right time."
Nine of the top eight seeds in the men's draw have now been eliminated.
Coria is now certain drop out of top five in the world rankings and be replaced by Tim Henman after the Briton won his third round tie 6-3, 6-1 against another Argentine, Juan Ignacio Chela.