World number one Iga Swiatek qualified for the semi-finals at the WTA Finals in Fort Worth, Texas on Thursday after Daria Kasatkina beat American Coco Gauff 7-6(6) 6-3 in a round-robin stage match at the eight-player, season-ending tournament.
Swiatek, who improved to 2-0 in this year's competition following her own 6-3 6-2 win over Caroline Garcia, goes through as the winner of her group, while Gauff can no longer qualify for the semi-finals.
Gauff started the match strongly, roaring to a 4-1 lead, before Kasatkina fought back to level the first set at 5-5. Gauff's eyes welled up with tears during the changeover after she was then broken to fall behind 5-6.
The 18-year-old regrouped to break back and force a tiebreak, but Kasatkina once again showed grit to fight back from 3-0 down and claim the breaker 8-6.
Kasatkina, a WTA Finals debutante, kept up the pressure in the second set, trading a couple of breaks with Gauff, before breaking to take a 4-2 lead and then serving out the match a few games later.
"To win for the first time at the Finals is an incredible feeling," Kasatkina said.
"Obviously you want to win and go further in the tournament, so the motivation to win is there, but it's not easy to manage the nerves and I feel it. I hope I can make it (to the semi-finals) in the next match."
For Swiatek, who upped her career Grand Slam tally to three with wins at this year's French Open and U.S. Open, her victory over Garcia earlier on Thursday gave her some measure of revenge on the Frenchwoman after having fallen to her in the quarter-final at the Poland Open in July.
The 21-year-old Pole broke at love for a 5-3 lead and then served out what proved to be a tight opening frame on her third set point when world number six Garcia sent a forehand long.
Swiatek then broke Garcia to start the second set and in the next game calmly roared back from 0-40 down to hold serve and never faced a break point the rest of the way in a match she sealed in style with an ace.
Garcia tried to use aggressive court positioning to throw her opponent off and was often returning serves from inside the baseline but Swiatek was prepared for the Frenchwoman's tactics.
"Yeah, she was putting a lot of pressure and putting a lot of speed on her balls so I needed to get them back and hit them even better. I was ready for that," Swiatek said during her on-court interview.
"We had really good tactics with my coach and they gave me the belief that I could dominate even though she is playing really, really well."