CANCUN, Mexico -- No.2 Iga Swiatek and No.3 Coco Gauff secured their spots in the semifinals of the GNP Seguros WTA Finals Cancun on a wind-strewn Friday evening.
Gauff won a battle of two of the Grand Slam champions from this season at the GNP Seguros WTA Finals, defeating Marketa Vondrousova 5-7, 6-7(4), 6-3 in the final round of group play on Friday night.
The win eliminated the Wimbledon champion's hopes of advancing to the semifinals. Gauff is the first teenager to make the final four of the year-end championships since Caroline Wozniacki in 2009.
Later in the evening, Swiatek confirmed her position as the group winner after defeating No.6 seed Ons Jabeur 6-1, 6-2 to go 3-0 in the Chetumal Group.
On Saturday, Swiatek will face No.1 Aryna Sabalenka in a match that could decide the year-end No.1 ranking. If Sabalenka wins, she will finish the year at No.1. If Swiatek prevails, she can retake the top spot only by winning the title. This is a rematch of last year's semifinal in Fort Worth, where Sabalenka won 6-2, 2-6, 6-1. The two have split their two meetings this season, both on clay. Swiatek won the Stuttgart final, while Sabalenka took Madrid.
"We'll see what the conditions are going to be," Swiatek said. "Against Aryna it's never easy. I'm just gonna do my best and focus on myself and what I want to do on court. We usually have these matches that are really exciting and really tough, both mentally and physically."
The other semifinal will showcase Gauff against No.5 Jessica Pegula. It will be the first all-American semifinal at the WTA Finals since the reintroduction of the round-robin format in 2003. Both players are making their semifinal debuts at the year-end tournament. Last year in Fort Worth the duo went winless in both singles and doubles. This will be the third meeting of the year between the two top-ranked Americans. Gauff won on the grass in Eastbourne and Pegula got her revenge during her title run in Montreal this summer.
Saturday's semifinal schedule, which includes one remaining round-robin doubles match that will determine the fourth semifinalists:
 Gabriela Dabrowski & Erin Routliffe vs.  Nicole Melichar-Martinez & Ellen Perez
NB 5:00 p.m.
 Jessica Pegula vs.  Coco Gauff
NB 6:00 p.m.
 Aryna Sabalenka vs.  Iga Swiatek
 Coco Gauff & Jessica Pegula vs.  Laura Siegemund & Vera Zvonareva (to finish, 6-3, 1-1)
After suitable rest
TBC vs.  Storm Hunter & EliseMertens
In a match with multiple interruptions because of intermittent rain, Gauff improved her record over Vondrousova to 3-0. As the players adjusted to the conditions, Vondrousova led by a double-break at 3-0 in the first set and held off the American's comeback attempt to take it 7-5.
Gauff looked on her way to quickly taking Vondrousova into a deciding set after dominating the first five games of the second to lead 4-1. But after failing to serve out the set at 5-3, Gauff found herself four points from defeat. With Vondrousova serving for the win at 6-5, the American threw caution to the wind, playing aggressive tennis to overpower Vondrousova and take the match to a third set.
"I felt like I needed to hit through the court," Gauff said. "She's a very tricky player. When you play her pace and let her move you side to side it's tough. And I played her, it's my third time, so I know that.
"But obviously, in the wind, it's tough to be as aggressive as you want, because the bounces are weird and the ball was moving. So I'm glad I made that adjustment and didn't get too frustrated when I missed. You pay the price for hopefully a good reward."
Gauff's aggressive game plan, combining opportunistic forays into the net and flatter hitting from the baseline, continued in the final set. After breaking at love for a 3-2 lead, Gauff kept Vondrousova at bay to finish with a 2-1 record in the Chetumal group.
The victory is Gauff's 51st of the season. She is the first American teenager to register 50 or more wins in a single season since Venus Williams in 1999. She has won 24 of her last 27 matches, with those three losses coming to fellow semifinalists Swiatek (2) and Pegula.