Angelique Kerber maintained her 100 per cent record at the WTA Finals by beating Madison Keys 6-3 6-3 to ensure Dominika Cibulkova will join her in the last four.
Kerber was already assured of a place in the semi-finals of the lucrative season-ending tournament in Singapore for the first time as a result of Cibulkova's straight-sets victory over Simona Halep, which knocked the third seed from Romania out.
Keys would have qualified from the Red Group along with Kerber if she avoided a straight-sets defeat, but the clinical world number one did Cibulkova a huge favour with a comfortable win.
Cibulkova lost her first two matches this week, but defied the odds to prevail at the expense of fellow debutant Keys.
Kerber was not at her best, but the top seed clearly had no intention of preserving energy for her semi-final as she attempts to end a stellar year on a high note.
The Australian and US Open champion gained the upper hand by breaking to lead 2-1, but let her advantage slip with a double fault in a strange first set and let Keys back in immediately for a second time to be pegged back at 3-3.
A fierce forehand winner from the German put her back in front at 4-3 and she showed great defence to earn three set points, winning the second when Keys volleyed into the net.
The sixth seed made a promising start to the second by taking a 2-0 lead, but Kerber turned the tide to win five games in a row and Keys' season came to end when she drilled a return into the net.
Angelique Kerber  bt Madison Keys  6-3 6-3
Kerber - 32/27
Keys - 10/13
Kerber - 3
Keys - 0
BREAK POINTS WON
Kerber - 6/7
Keys - 3/4
FIRST SERVE PERCENTAGE
Kerber - 55
Keys - 60
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Kerber - 62/52
Keys - 51/20
Kerber - 61
Keys - 42