Singapore - Germany's Angelique Kerber continued her unbeaten run at the WTA Finals on Thursday, steamrolling rising American Madison Keys 6-3 6-3 to charge into the semi-finals of the season-ending championship.
Still on a high after winning the US Open last month, the newly crowned world number one made it three wins on the trot at Singapore's Indoor Stadium to finish at the top of the Red Group standings after the round-robin phase.
Kerber was joined in the semis by Slovakia's Dominika Cibulkova, who claimed her first victory of the tournament by defeating Simona Halep of Romania 6-3 7-6 (7-5), to finish as the group runner-up.
Beaten in her first two matches, the 27-year-old Cibulkova needed to see off Halep in straight sets then rely on Kerber defeating Keys in straight sets to advance, but she made it through when everything she hoped for went according to plan.
"This win is really, really big for me," Cibulkova said. "I feel relieved. I'm really happy about my win, about the way I played and the way I handled the situation and the pressure.
"It seems like I'm playing my best tennis under the biggest pressure. It's just a great feeling to beat Simona in two sets knowing that if I want to go further in the tournament I have to beat her in two sets. That made me even stronger in the second set."
Cibulkova will play Svetlana Kuznetsova in Saturday's semis after the Russian finished top of the White Group. Kerber will meet with either the defending champion, Agnieszka Radwanska, or US Open finalist Karolina Pliskova, who meet on Friday to decide the last available spot.
Both Keys and Halep would have advanced ahead of Cibulkova if either could have won a set on Thursday but both women were outplayed by their opponents.
The big-hitting Keys had her serve broken six times by Kerber and contributed to her own downfall with 31 unforced errors.
"This is my first experience at a setting like this and I think my nerves got the best of me," Keys said.
"I think that happens to everyone and it's just something I need to work on. It's not a bad thing, it happens to a lot of people."
Halep, a runner-up at the elite eight-player event two years ago, was philosophical about her defeat, saying she was pleased just to make the tournament after a slow start to the year.
"This tournament was a bonus for me," she said. "I gave it everything I had. I didn't have enough to qualify in the semis, but it was a good experience again, my third year in a row, so it's a good thing.
"I'm not sad that I lost. Of course it's tough, but nothing changes."
Results from the WTA Finals tournament in Singapore on Thursday (x denotes seeding):
Round-robin stage Red Group
Dominika Cibulkova (SVK x7) bt Simona Halep (ROM x3) 6-3 7-6 (7-5)
Angelique Kerber (GER x1) bt Madison Keys (USA x6) 6-3 6-3