Williams beat Katarina Studenikova, of Slovakia, 6-1, 6-0, and Seles also dropped only one game as she overcame Silvia Farina, of Italy, 6- 0, 6-1.
The two Americans were joined in the semi-finals by two French players, the fifth-seeded Mary Pierce and the seventh-seeded Sandrine Testud. Pierce beat unseeded Magui Serna, of Spain, 7-5, 6-3, while Testud forced her way past the third-seeded Conchita Martinez, of Spain, 7-6 7-5.
Williams now plays Pierce with Seles facing Testud.
Pete Sampras pulled out of the Lyon tournament yesterday because of a back injury that threatens his attempt to retain the world No 1 ranking for a record sixth successive season.
Sampras started complaining about his back during his first-round victory over his compatriot Todd Martin on Wednesday. Hours before his scheduled quarter-final against the German Tommy Haas yesterday, the American realised he could not go on.
"I felt a pain when I served in the first set against Todd. My back was still stiff when I played against Todd Woodbridge and I realized this morning that I could not keep going on like this," Sampras said.
In his bid to stay No 1, Sampras had decided to play five tournaments in a row, something he had not done since 1993.