LINZ, Austria — Top-seeded Angelique Kerber defeated Patricia Mayr-Achleitner of Austria 6-3, 6-1 in the quarterfinals of the Generali Ladies on Friday to secure the final spot in the season-ending WTA Championships.

"Qualifying for Istanbul is a great achievement," Kerber said. "It's the reward for my consistency, though I've had my ups and downs and a couple of injuries this year."

The 10th-ranked German will play fourth-seeded Carla Suarez Navarro of Spain. Two-time former champion Ana Ivanovic will take on Stefanie Voegele of Switzerland in the other semifinal. The 58th-ranked Voegere advanced after upsetting second-seeded Sloane Stephens of the United States 6-7 (3), 6-4, 7-5.

The 12th-ranked Stephens lost the last five games of the final set. Stephens also lost a 5-2 lead in the opening set before converting her ninth set point in the tiebreaker. Voegele used one break to take the second set and won the third after trailing 5-2.

"To come back from 2-5 in the final set is incredible," said Voegele, who reached her third semifinal of the season. "I told myself to keep on fighting, and I am proud that I was able to win."

Ivanovic defeated seventh-seeded Dominika Cibulkova of Slovakia 6-2, 6-4. The third-seeded Serb, who won the event in 2008 and '10, was 3-0 down in the second set before reeling off four straight games. Ivanovic is yet to reach a final this season.

Suarez Navarro beat fifth-seeded Kirsten Flipkens of Belgium 6-1, 6-3 to reach her fourth semifinal of the season and first on hardcourt. Suarez Navarro won the first eight points of the match and held serve throughout.

Kerber saved a break point while trailing Mayr-Achleitner 3-2 in the opening set. She then won nine straight games to lead 6-3, 5-0 before closing out the victory on her first match point.