Two-time Grand Slam champion Garbine Muguruza booked her semi-final spot at the WTA Finals on Sunday with a 6-4, 6-4 victory over Anett Kontaveit that ended the Estonian's 12-match win streak.
Muguruza's victory in her final round-robin contest in the eight-woman season finale in Guadalajara denied Karolina Pliskova a place in the knockout phase.
The former world number one Pliskova battled to a gutsy 0-6, 6-4, 6-4 victory over fellow Czech Barbora Krejcikova earlier in the day.
Muguruza will face fellow Spaniard Paula Badosa on Tuesday.
"It's amazing there are four players left and two of them are Spaniards. I mean, that just shows that Spain has a great level of tennis and a great school. It's going to be a fun match," Muguruza said.
Kontaveit was already assured of a semi-final berth after winning her first two matches to stretch a WTA winning streak that included titles at Moscow and Cluj-Napoca.
But it was a must-win match for Muguruza after Pliskova's afternoon victory, and the Spaniard came out firing, breaking Kontaveit in the opening game.
Muguruza saved the only three break points she faced -- two as she served for the opening set and one more as she served for the match.
She exploited Kontaveit's forehand struggles, taking the first set on two forehand errors from the Estonian.
Serving for the match at 5-4 in the second she saved a break point with an her seventh ace and claimed the match after one hour and 27 minutes with a service winner to finish second in the group behind Kontaveit.
That ended the hopes of Pliskova, who rallied from a set and a break down to beat reigning French Open champion Krejcikova.
Pliskova fired 11 aces but almost undid that good work with 12 double faults.
She saved two break points in the penultimate game of the second set and broke to force the decider, which she won on her third match point.
"I just fought hard to give myself a chance," Pliskova said,
"Because she didn't really miss anything the first set. Happy I stayed in there and just waited for the chance."