Memphis, TN (Sports Network) - Top seed Kirsten Flipkens went by way of the upset, while former top-20 player Sabine Lisicki was an easy quarterfinal winner Thursday at the $235,000 U.S. National Indoor Tennis Championships.

Seventh-seeded Slovak Magdalena Rybarikova, who captured this event two years ago, upended the Belgian Flipkens 6-7 (11-13), 7-5, 6-3, while the third- seeded German slugger Lisicki subdued eighth-seeded Frenchwoman Kristina Mladenovic 6-3, 6-3 on the hardcourts at The Racquet Club of Memphis.

Another mild upset came when Swiss Stefanie Voegele whipped fourth-seeded Brit Heather Watson 6-1, 6-2, while one other quarterfinal saw last year's Memphis runner-up Marina Erakovic of New Zealand douse American Jamie Hampton 7-5, 6-2.

Erakovic lost to Swede Sofia Arvidsson in last year's finale here.

Friday's semifinals will pit Lisicki against Rybarikova and Voegele versus Erakovic.

02/21 15:26:57 ET