Venus Williams beat Victoria Azarenka in a clash of former world number ones to reach the last eight at the Bank Of The West Classic in Stanford, USA.
The American, 34, won 6-4 7-6 (7-1) to reach her third quarter-final of the year and stay on course for a possible semi-final against her sister, Serena.
Venus, ranked 25th, will face Germany's Andrea Petkovic in the last eight.
"Williams-Williams tomorrow, hopefully," she told the crowd after her win on Thursday.
The result means Belarusian Azarenka, who was playing on her 25th birthday, will drop out of the world's top 10 in a season disrupted by three months out with a foot injury.
Serbia's Ana Ivanovic is heading in the opposite direction and her 6-1 6-1 win over Canadian qualifier Carol Zhao guarantees a return to the top 10 next week after an absence of more than five years.
The former world number one and French Open champion will take on Serena Williams, who currently tops the rankings, in the quarter-finals on Friday.
Ivanovic beat the American at the Australian Open in January, but Serena won their last meeting in Rome three months ago.
"It's a good sign," said the Serb, 27. "It means we're getting further into the tournaments to play each other."