Stephens beat Kiki Bertens to improve to 2-0 in group play, while Kerber's win over Naomi Osaka brings here to 1-1.
SINGAPORE — Sloane Stephens and Angelique Kerber both earned three-set victories at the WTA Finals on Wednesday to leave their group wide open ahead of the last round.
Stephens, who is making her debut at the year-end event, beat Kiki Bertens 7-6 (4), 2-6, 6-3 for her second straight victory in round-robin play.
Earlier, Kerber held her nerve and her serve to beat Naomi Osaka 6-4, 5-7, 6-4 after saving 13 of the 18 break points she faced, including five in the final game of the first set.
The results mean all four players could still advance to the semifinals. Stephens is the only player with a 2-0 record, with Kerber and Bertens at 1-1 and Osaka dropping to 0-2.
Stephens would have secured a semifinal spot if she had won in straight sets but Bertens managed to extend the match past midnight local time.
''Thank you so much for staying as I know it's after everyone's bedtime,'' the American told the crowd.
Kerber had a chance to win straight sets, but the Wimbledon champion was broken while serving at 5-4. In the final set, the German didn't squander a second chance and an error from Osaka on match point ended it.
Osaka, who won this year's U.S. Open, finished the match with 50 unforced errors and 42 winners, while Kerber had 30 unforced errors and 24 winners.
''I feel like this match was sort of a continuation of my last match in a way that I feel like I just didn't serve well, in either of them,'' Osaka said. ''I'm playing against the best players in the world, so I feel like I want to be in top form, but I'm not really there.
''So it's a little bit depressing.''