Maria Sakkari fended off a second-set fight back from Aryna Sabalenka to emerge with a 6-2 6-4 victory and reach the semis of the WTA Finals in Fort Worth on Wednesday, while Ons Jabeur overcame a sluggish start to beat Jessica Pegula.
Sakkari cruised through the first set and led 3-0 in the second before the hard-hitting Belarusian roared back, winning four straight games to take a 4-3 lead.
But the determined Greek reeled off the final three games and raised her arms in triumph when Sabalenka's running forehand landed wide on the purple indoor hard courts in Texas.
"I felt like I did nothing wrong from 3-0 down," Sakkari, who reached the semi-finals of the tournament for a second consecutive year, told Tennis Channel.
"She started making more balls, she wasn't missing like she was in the first set and half. So I just told myself, keep doing what you're doing and you'll get your chances." Sakkari also benefited from some net cords that went her way in the rollercoaster second set.
"Sometimes it's nice to have a little bit of luck, and I'm usually not that lucky on the court," she said with a smile.
Earlier in the day, world number two Jabeur battled back to beat American Pegula 1-6 6-3 6-3 for her first win in round robin play.
Pegula came out firing against the Tunisian, catching up to a drop shot in plenty of time to deliver a backhand winner to snag the 25-minute first set.
But Jabeur came alive in the second, breaking Pegula when her forehand hit the net cord and landed out of bounds for a 4-3 lead en route to leveling the contest at a set apiece.
Frustration appeared to mount for Pegula in the decider and she did not even attempt to run down a Jabeur drop shot on break point that gave the Tunisian a 5-3 lead. Jabeur went on to seal the win on an errant backhand from Pegula on match point.
"In the first set, she was playing really well and really fast," Jabeur said.
"The balls were really tough, very low, and I know she likes to play like that. I just had to change up the rhythm and impose my game." Jabeur, who fell to Sabalenka in a tight three-setter in their opening match on Monday, said the round robin format meant she had no time to sulk over the loss.
"I'm used to being depressed for the next two days when I lose but I didn't have much time here," Jabeur said after collecting her 47th win of the season.
The WTA Finals features eight women split into two sections, with the top two finishers in each group moving onto the semis.
Sakkari is the first player from her group through to the last four. Jabeur, Sabalenka and Pegula are vying for the second spot.
The group featuring Iga Swiatek, Caroline Garcia, Coco Gauff1 and Daria Kasatkina returns to action on Thursday.