Carolina: Serena and Venus Williams set up a tantalising semi-final against each other after both won twice on Friday in the rain-affected Family Circle Cup in Charleston, South Carolina.
The American sisters will face off for the first time since the 2009 Sony Ericsson Championships.
But second seed Caroline Wozniacki will be able to head to Augusta National early to watch boyfriend Rory McIlroy contest The Masters next week after being bundled out by Switzerland's Stefanie Voegele 3-6 6-4 6-3 in the late quarter-final.
With their third round matches postponed from Thursday due to rain, the Williams sisters both faced a full schedule on Friday.
Venus started proceedings with a 6-2 4-6 6-2 win over fellow American Varvara Lepchenko while top seed Serena disposed of American qualifier Mallory Burdette 6-4 6-2.
Madison Keys earned a quarter-final match up with Venus after getting by wildcard Bethanie Mattek-Sands 7-6 6-4 in another all-American clash while sixth-seed Lucie Safarova of the Czech Republic set up a showdown with Serena when outlasting Romanian Sorana Cirstea 6-3 5-7 6-1.
Venus duly despatched Keys 6-4 6-4 in just 70 minutes in the afternoon while Serena beat Safarova 6-4 6-1 to extend her winning streak at the event to 13 matches.
"For the record, we've never played two matches on the same day," Serena told the stadium crowd. "You guys had a real treat."
"I was thinking it was a quadruple-header," Venus added. "I don't know if that's ever going to happen again."
While Serena holds a 13-10 lead in matches against Venus, the sisters have only played on clay twice with each sharing a win.
Serena won the title in her last two appearances in 2008 and 2012 while Venus won the event in 2004.
In the other quarter-final, Serbian former world number one Jelena Jankovic halted Canadian qualifier Eugenie Bouchard's hot run with a 6-2 6-1 demolition and posted the 500th win of her professional career.