Defending champion Caroline Wozniacki was knocked out of the WTA Finals at the group stage on Thursday after Elina Svitolina edged a thrilling encounter in Singapore.
The Ukrainian sixth seed outlasted Wozniacki 5-7, 7-5, 6-3 in two hours and 35 minutes to finish top of White Group.
Karolina Pliskova also qualified after the Czech defeated compatriot Petra Kvitova in straight sets earlier in the day.
Having won her opening two matches, Svitolina needed just one set to clinch her semi-finals spot, while Wozniacki required a straight sets victory.
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After the Australian Open champion claimed the much-needed first set, it all came down to a pulsating 12th game of the second.
Svitolina wasted four set points but an errant forehand from Wozniacki on her fifth triggered elation from the Ukrainian.
"It was an amazing match, she really pushed me to the limit," Svitolina said after the match. "I tried to stay aggressive and focused on placing the ball in the right corners."
Earlier, Pliskova became the first player into the final four after a 6-3, 6-4 triumph against Kvitova in 83 minutes.
Kvitova needed to win in straight sets to keep her campaign alive but struggled throughout to complete a disappointing first WTA Finals appearance since 2015.
Pliskova booked her spot in the semi-finals for the second straight year having also beaten Wozniacki earlier in the tournament.
"It was a huge victory for me," she said. "It meant a lot to get into the semi-finals after beating her, it's just special."
Kvitova played down fatigue as a factor after a gruelling season.
"I was really feeling well physically and mentally," she said. "I still had a lot of positive thoughts in the match to fight and turn it around but it didn't happen."
The final round-robin matches in Red Group will be played on Friday with all four players still in the race for semi-final spots.
US Open champion and third seed Naomi Osaka takes on No 8 seed Kiki Bertens, before top seed Angelique Kerber faces fifth seed Sloane Stephens.
