ZHUHAI, China, — Kiki Bertens topped her round robin group in convincing fashion with a 6-4, 6-3 victory over Dayana Yastremska to become the first player to qualify for the semifinals of the WTA Elite Trophy in Zhuhai, China on Thursday.

Both players had already beaten Croatia's Donna Vekic earlier this week, which made the final round robin match a virtual quarterfinal where the Dutch top seed got the better of the 19-year-old Ukrainian.

Yastremska's poor service game in the opening set — she made four double faults in three games — opened the door for Bertens who broke her teenaged opponent five times in the encounter.

Although Yastremska also converted two break points, she was no match for Bertens' patient baseline game which forced the Ukrainian to make numerous errors and a fifth double fault on match point ended the contest.

"Changing the rhythm, I think that's what she doesn't like," Bertens told reporters. "She really likes that every ball is coming at the same height, at the same pace.

"So just throwing in some slices -- I think she's really struggling with that, I saw that already in Beijing and I think today again. That's what I normally do already, but I think that against players like Dayana, even more."

Earlier, the Czech Republic's Karolina Muchova fought back from a set down in her round robin opener to beat American Alison Riske 2-6, 6-2, 7-5.

Muchova, who converted six of 13 break points, was 3-1 down in the deciding set before clawing her way back into the tie to keep her semifinal hopes alive.

Meanwhile, American third seed Madison Keys overcame fifth seed Petra Martic of Croatia 6-3, 6-4.

Fourth seed Aryna Sabalenka beat doubles partner and Belgian sixth seed Elise Mertens 6-4, 3-6, 7-5, to top their group and qualify for the semifinals of the WTA Elite Trophy in Zhuhai, China on Friday.

Despite making 10 double faults in the match, Belarusian Sabalenka — who has won three of her four singles titles in China — advanced after a physically taxing battle in two hours and 20 minutes.

Sabalenka will play the Czech Republic's Muchova, who fought long and hard for a second consecutive day and eventually came out on top with a 6-4, 4-6, 6-3, win over second seed Sofia Kenin to book her spot in the semis.

Muchova, who said she fought jet lag and a bout of dizziness in the second set, fired six aces and broke the American three times to win her second consecutive three-setter at the tournament.

China's 12th seed Zheng Saisai lost 6-4, 6-3 to Croatian fifth seed Martic in their final round robin match but the local favorite still qualified for the semis.

With Zheng, Martic and Madison Keys winning one match each in their group, the Chinese 25-year-old needed to win at least seven games to advance if she were to end up losing the final match.

"This is the first time I've had this experience," Zheng said. "Actually, I knew if I win a set, I for sure would be in, but I don't know exactly how many games.

"So I was pretty tight because I knew I don't need to win the match. I was scared a little bit... very nervous. But she was pretty sure. She said, 'congratulations' and I was like, 'Oh, okay, that's great.'"

She plays top seed Kiki Bertens in the other semifinal after the Dutchwoman advanced on Thursday. — Reuters