MEMPHIS, TN, USA - Sometimes in tennis, one bad game can prove the difference between winning and losing.

For Kirsten Flipkens, that game came at 6-5 down in the second set of her US National Indoor Tennis Championships quarterfinal with Magdalena Rybarikova.

Having pipped Rybarikova to the opening set on an epic tie-break, the second appeared to be heading in the same direction. But, at this least opportune of moments, the top seed's previously unflappable serve faltered as Rybarikova broke to love to force a decider.

"I think the difference was in the second set when she was leading 6-5, I had a bad game," Flipkens said. "It's a shame because you're fighting for every point and then you have a bad game and the set is finished."

Unfortunately for the Belgian, half an hour later, history repeated itself; another loose service game, this time at 4-3 down, allowed Rybarikova to capture the vital break before serving out for a 67(11) 75 63 victory moments later.

"I'm very happy that I won this match," Rybarikova, the 2011 Memphis champion, said. "I really like to play indoors, and especially this tournament, and it feels good to be in the semifinals again.

"It wasn't an easy match today. Kirsten is a very tough player who is playing very well at the moment and she made me fight all the way."

Meeting Rybarikova for a place in the final will be No.3 seed Sabine Lisicki, who produced her best display of the week to defeat Kristina Mladenovic, 63 63.

"I played a really good match and I tried to be as aggressive as I could in my game and it worked really well," Lisicki said. 'I think it was one of my best matches of the season and I hope I can be even better on Friday."

The other semifinal will pit last year's runner-up, Marina Erakovic, against the tournament's surprise package, Stefanie Voegele.

At No.89 in the world, Voegele was the lowest-ranked player in the last eight, but she made a mockery of the tennis hierarchy to oust No.4 seed Heather Watson, 61 62. Erakovic was nearly as impressive, seeing off the last American standing, Jamie Hampton, 75 62.

"Today was really tight," Erakovic said. "I played Jamie earlier this year and she beat me, so I knew it was going to be tough.

"I think in the first set, it was a point here or there that decided it. I got a bit of confidence from winning that set, which gave me a head in the second, and managed to pull it out. It was high-quality tennis so I'm very happy I got through."