In a match between the last two US Open champions, Sloane Stephens came out on top.
The 2017 champion beat Naomi Osaka 7-5, 4-6, 6-1 on Monday at the WTA Finals, the season-ending tournament for the top eight players in the world.
Both players are making their debut appearances at the tournament, but Stephens appeared to be more composed while Osaka, who became the first Japanese citizen to win a Grand Slam title last month in New York, never seemed comfortable.
"I'm just really happy to get through and play a good competitive match," Stephens said on court after the match. "I never gave up. I knew she's been playing well and I'd have to play really good tennis to beat her."
Both players struggled to hold serve. Osaka was broken on seven of 19 break-point opportunities, while Stephens was broken on four of 12.
Stephens entered the tournament with 33 wins this season, the least of any of the eight qualifiers. She reached her second career Grand Slam final at this year's French Open, and won the Miami title.
Osaka continually showed her frustration during the match. When she failed to capitalize on four break points in the fourth game of the second set, she made a visible shrugging gesture with both hands.
Stephens gifted the second set to Osaka when she double-faulted on a second set point in the 10th game. But Osaka appeared to be mentally fatigued and only managed to hold serve in the third game of the final set.
Stephens is now 1-0 in the Red Group, while Osaka is 0-1.
In the other game of the day, Wimbledon champion Angelique Kerber lost to 6-1, 3-6, 4-6 Kiki Bertens, who are also in the same group.
federer reveals injury
Roger Federer has revealed that he has been suffering with a hand injury since the summer, but said the problem is no longer a worry as he heads into his home Swiss Open starting on Monday.
The 20-time Grand Slam winner told Germany's Sonntag Zeitung of the problem which began when he trained for the grasscourt season.
"I hurt my hand training at the start of the grass season," Federer said. "It's had more consequences than I thought. I dragged this pain for about 3 months.
"It's not an excuse and we're not going to make a fuss over it."
He added: "(Sometimes) I felt pain during the first ten minutes of a match warmup. But now I can let go of my forehand normally without thinking of it."
The 37-year-old Federer will start his bid for a ninth Basel title on Tuesday when he plays Serbia's Filip Krajinovic and could face a revenge match in the second round if he and John Millman both advance. The 33rd-ranked Australian shocked Federer in the US Open 4th round.