SINGAPORE -- In the opening round-robin match for two first-timers at the BNP Paribas WTA Finals Singapore presented by SC Global, No.5 seed Sloane Stephens of the United States overcame No.3 seed Naomi Osaka of Japan, 7-5, 4-6, 6-1, in the first match in the Red Group on Monday.

"I thought I played a really good match," Stephens said in her post-match press conference. "I played solid from start to finish. Obviously playing someone with a lot of confidence is always a tough task, but I think that I played well and I kept a good attitude. So I was pleased with that."

The big-hitting affair between the two Singapore debutantes was a lengthy clash, finishing after two hours and 22 minutes of engrossing play, which saw 2017 US Open champion Stephens extend her head-to-head record to 2-0 over the reigning US Open champion.

"I knew from the very beginning it was obviously going to be a tough match, so I was going to have to stay in a lot of long points and make sure that I made a lot of plays on the ball, which I did," Stephens stated. "I thought when she was hitting good shots, I got my racquet on a lot of the balls and I just made her play an extra ball."

"I can't be upset with how I played," Stephens continued. "We played a lot of great long rallies, and she played a lot of great short points. I think all around we played a good match."

Stephens, who won the Miami Open and reached her second Grand Slam final at Roland Garros this year, broke Osaka seven times out of 19 break points, winning nearly two-thirds of points when returning her opponent's second serves. Osaka, who also triumphed in the Indian Wells championship alongside her US Open title in 2018, fired seven aces but was undone by 46 unforced errors.

Either top seed Angelique Kerber or 2018 WTA Most Improved Player Kiki Bertens will join Stephens at the top of the Red Group table, as they will face off in a later round-robin encounter on Monday. Bertens is also making her WTA Finals debut, meaning World No.2 Kerber must battle three debutantes during the round-robin portion of the event.

After a nervy exchange of breaks to open the match, the players settled into solid play on serve, comfortably holding through to 3-3. Stephens then broke an-error prone Osaka to jump ahead 4-3, but she failed to consolidate from a positive position, double faulting twice in a row from 40-30 to hand Osaka a break point. Osaka converted it with a forehand winner into the corner, leveling the set at 4-4.

But Stephens kept having Osaka’s number on serve in the opening set. Osaka used powerful serving to erase a break point at 4-4, but Stephens got another chance to break at 5-5, which she claimed after a rally ended with Osaka plopping a backhand miscue into the net.

Stephens needed three set points to close out the set at 6-5, but Osaka fired an error into the net at the end of a rally on Stephens's third opportunity, this time off the forehand, to put the American up a set. Osaka had 15 unforced errors in the first set, and could only manufacture three winners off the ground during that timeframe.

The American seemed to be pulling the match her way at the start of the second set, overcoming four break points with outlandishly incredible defense en route to a hold for 2-2. Stephens then had two break points in the next game before Osaka fired a number of winners from her backhand side to hold for 3-2.

It was the backhand which served the Japanese player well in the opening stages of the second set, and she used it to keep the following game close. Suddenly, Stephens double faulted on break point to provide Osaka with an imposing 4-2 lead. Osaka nearly ceded the break back, but staved off a break point with a great serve, eventually holding for 5-2.

Serving for the set at 5-4, Osaka faltered, blasting a forehand swing volley into the net to drop the game. But the Stephens double faults came back with a vengeance, as the American hit two in the next game, including on Osaka’s second set point, to tie up the match at one set apiece.

But it was Stephens who regrouped and dominated proceedings in the deciding set, as the American steeled herself while Osaka's errors mounted dramatically. A backhand service return winner into the corner on Stephens's third break point of the opening game determined the tone of the decider, and after holding for 2-0, saving a break point in the process, the American was on her way to the win.

Osaka saved a break point to hold for 2-1, but that would be the extent of her success in the final set, as the Stephens defense was too much for Osaka as the match progressed. A sterling running crosscourt forehand by the American forced an error on the third break point of Osaka's next service game, giving her a 4-1 lead. A hold at love for 5-1 then put Stephens firmly in the driver's seat.

Serving to stay in the match, Osaka saved one match point with an ace, but the Japanese player double faulted on the American's second match point, to put Stephens up with a 1-0 match record in the Red Group, after the first day of their group's encounters.