Zhuhai - Defending champion Julia Goerges beat Elise Mertens on Thursday to book her place in the semi-finals of WTA Elite Trophy.
The fifth seed bounced back from a bruising defeat to Anett Kontaveit on Wednesday, but overcame Mertens 6-2, 7-6 in a tense round-robin tie in Zhuhai, southern China.
Goerges, 29, was pushed all the way in the second set by her 22-year-old Belgian opponent, but the German clinched the nail-biting tiebreak 7-5 at the Hengqin International Tennis Centre.
The straight sets victory was enough to take her to the top of the group, and the world number 14 jumped with joy and punched the air in celebration.
Earlier, Australia's Ashleigh Barty beat an out-of-sorts French number one Caroline Garcia to keep her semi-final hopes alive.
Ninth seed Barty broke Garcia once in each set to win the round-robin tie 6-3, 6-4 in one hour and 16 minutes.
Barty, who lost to Aryna Sabalenka on Tuesday, now has to wait for the outcome of Garcia and Sabalenka's match to see if she will progress to the semi-finals on Saturday.
"It's out of my hands," Barty said in her post-match press conference.
"I just have to sit back and watch tomorrow. But I'm certainly pleased to have had the year we've had and really nice to finish on a positive note today."
She said she was in a "win-win situation", being able to "play with freedom", but knowing that if she lost she could still go home at the end of the season.
Garcia, 25, was crowned champion in Tianjin in October, but she was unable to bring that form into her opening match.
The world number 18, seeded eighth, said she had some back pain coming into the tournament, and felt her legs weren't moving the way she wanted on court.
"It was a very difficult match for me today and I think she played great but I wasn't feeling very good and didn't have the rhythm I wanted.
"It's a difficult start for me of course, but I was happy to start on Thursday because I had some back pain when I got back to France last Monday so it was a very complicated week for me.
"Today I think Ash played a great match as well."
The Elite Trophy features 12 players ranked roughly from nine to 20. This year's total prize money is $2.35 million across the singles and doubles tournaments.