Serena Williams produced a performance of the highest quality on Thursday when she beat Karin Knapp in straight sets in the third round of the Western & Southern Open in Cincinnati.
After something of a lacklustre showing in the second round against Tsvetana Pironkova, the world number one dominated the rallies and served impeccably against Knapp to win 6-0, 6-2.
Williams' opponent in the semi-finals is former world number one Ana Ivanovic, after the ninth-seeded Serb outlasted American Sloane Stephens, winning 2-6, 6-4, 6-1 in a match that lasted just over two overs.
Ivanovic's Serbian compatriot Jelena Jankovic also advanced to the last eight after upsetting eighth seed Karolina Pliskova 6-2, 3-6, 7-5 in what proved to be another hotly-contested three-setter.
Earlier, French Open runner-up Lucie Safarova advanced after her opponent Belinda Bencic retired hurt with a forearm injury.