Venus Williams beat her sister Serena for the first time in three and a half years, sealing a place in the semi-finals of the Nasdaq-100 Open with a 6-1, 7-6 victory.
Venus brought to an end her sister's three-year reign as women's champion here and will play Wimbledon champion Maria Sharapova in the last four.
"We've played some huge matches," said Venus. "This one definitely is a highlight.
"The first set was fantastic. The second set I definitely missed some opportunities (but) credit to Serena. It's not like she gave me that many points. I definitely had my hands full."
Venus had lost to her younger sister in their last six meetings and had not bettered her since the 2001 US Open final.
"A loss is a loss, so for me it's no different to losing to any top player," said Serena.
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"There was nothing I could do in the first set but I had plenty of chances in the second," said Serena. "I made some errors at the wrong time."
Sharapova beat former world number one Justine Henin-Hardenne 6-1, 6-7, 6-2 in her quarter-final to end the Belgian's comeback run.
In the men's draw, six-times champion Andre Agassi went through to the last eight after beating French Open champion Gaston Gaudio 7-6, 6-2.
"A match like that can just turn into a first class battle or if you take your right chances, you can maybe break a match like that open," said Agassi, who will play Taylor Dent in the quarter-finals after the American beat German Florian Mayer 6-3, 6-4.
"Today, that's what happened."
Top seed Roger Federer survived a stiff fourth round test from Mario Ancic, eventually beating the Croatian 6-3, 4-6, 6-4 to record his 29th win of the season.
The Swiss has lost just one match since last August and next plays Tim Henman after the sixth seeded Briton beat Czech Radek Stepanek 7-5, 6-3.
BACK TREATMENT
Sharapova could have beaten Henin far more emphatically had she converted one of three match-points in the second set. After missing those she raised her game in the decider, powering to victory after two hours and 17 minutes.
"I knew I had to lift my game in the third set because all of a sudden she was getting a lot more balls back," said Sharapova. "She's a huge fighter out there."
Sharapova received treatment on her lower back after dropping the second set but played down the extent of the injury ahead of her clash with Williams.
"I've had a back problem for a little while already," Sharapova said. "I got some treatment and I just tried to forget about it. Hopefully I'll be back fresh."
Henin was playing her first tournament since last year's U.S. Open. She spent much of last season battling the virus mononucleosis and was sidelined for the first three months of this season with a knee injury.
In other men's fourth round action, Dominik Hrbaty beat French teenager Gael Monfils 6-3, 6-3 and will play David Ferrer in the quarter-finals. The Spaniard defeated compatriot Juan Carlos Ferrero 6-7, 6-3, 7-5.
Swede Thomas Johansson beat Czech Jiri Novak 7-6, 6-2 to set up a meeting with Spanish teenager Rafael Nadal, who needed three sets to get past Croatian Ivan Ljubicic 6-4, 6-7, 6-3.