Venus and Serena Williams will play against each other for the first time in more than three years in the semi-finals of the Family Circle Cup.
Both completed two matches on Friday to get the rain-affected event back on schedule in Charleston, South Carolina.
Defending champion Serena, who holds a 13-10 record against her elder sister, beat qualifier Mallory Burdette 6-4 6-2 and sixth seed Lucie Safarova 6-4 6-1.
Venus overcame Varvara Lepchenko 6-2 4-6 6-2 and Madison Keys 6-4 6-4.
The Williams sisters have not gone head-to-head on tour since Serena beat Venus in the final of the season-ending WTA Championships in 2009.
World number one Serena, 31, has won the last four meetings against 32-year-old Venus, who is now ranked 24th.
"It was really motivating to see Venus win today. I figured if she can win two matches, I have no excuse not to win my two matches," Serena said.
"Venus is the toughest opponent I've ever played. I think she's beaten me the most out of any player. I know her game and she knows my game. I think it'll be a great match."
In the other semi-final, ninth seed Jelena Jankovic will play the unseeded Stefanie Voegele after the 23-year-old Swiss beat second seed Caroline Wozniacki in three sets.
Wozniacki, the world number 10, had looked to be in charge of the match at various times, particularly when she led by a break of serve at 3-1 in the deciding set.
But Voegele, the world number 63, won five games in a row to secure a 3-6 6-4 6-3 victory, and a first meeting with 28-year-old Jankovic, who beat Canadian qualifier Eugenie Bouchard 6-2 6-1.