Simona Halep of Romania and Caroline Wozniacki of Denmark will meet in the semifinals of the Porsche Grand Prix after winning their respective quarterfinals on Friday in Stuttgart, Germany.
The second-seeded Halep needed 81 minutes to defeat Sara Errani of Italy, 6-4, 6-4, while the fourth-seeded Wozniacki had little trouble in defeating Carla Suarez Navarro of Spain, 6-0, 6-3.
Irrespective of the result in Saturday's semifinal, Halep will move up to No. 2 when the world rankings are updated Monday following
Kerber upset another Russian, the sixth-seeded Ekaterina Makarova, 6-3, 6-2, to book her place in the other semifinal, where she will face Madison Brengle of the United States.
The American, who upset third-seeded Petra Kvitova on Thursday, rallied to beat Caroline Garcia of France, 3-6, 6-3, 6-3, in the last quarterfinal.
Defending champion Kei Nishikori made his way into the semifinals of the Barcelona Open by holding off Roberto Bautista Agut, 6-2, 3-6, 6-1.
Third-seeded David Ferrer also advanced by beating Philipp Kohlschreiber, 6-3, 7-6 (5), as he stayed on course for a fifth final appearance at the tournament.
After the top-seeded Nishikori won the first set easily, Bautista Agut rebounded in the second and got his first break. He then saved three break points in his final service game before using two big serves to force the third set.
Nishikori re-established his ground game to take a 3-1 lead before winning the last three games.
Nishikori will next face Martin Klizan after the Slovak needed to receive a massage on his right forearm twice en route to eliminating
Klizan overcame his five double faults and Robredo's eight aces by saving all six break points.
Ferrer will meet Pablo Andujar after his countryman converted five of seven break chances to brush aside Italy's
Seventh-seeded Jiri Vesely of the Czech Republic reached his third ATP tour semifinal of the season by overcoming third-seeded
Vesely withstood 11 aces from the hard-serving Karlovic to wrap up the win in 79 minutes.
The 20-year-old Vesely, who won the Heineken Open in January, and will face the winner between top-seeded Gilles Simon and