SINGAPORE -- Singapore debutante Sloane Stephens completed her undefeated run through the round-robin portion of the BNP Paribas WTA Finals Singapore presented by SC Global, as the 5th-seeded American defeated No.1 seed Angelique Kerber of Germany, 6-3, 6-3, to win the Red Group and reach the semifinals.

"Playing Angie is always tough for me," Stephens stated, during her post-match press conference. "She's No. 2 player in the world. She's a tough match. Obviously coming here and playing against the top players in the world, nothing is going to be easy."

"I just prepared," Stephens continued. "Obviously a lot on the line today, trying to make the semis and trying to get through. I just tried to do my best and battled as hard as I could and stick to my game plan."

Live blog: As it happened: Stephens heads to semfinals after Kerber win

After dropping their first meeting at Indian Wells in 2012, Stephens has won her last five matches against Kerber, going 10-0 in sets in the process. Following the 100-minute victory, Stephens became the first WTA Finals debutante to go 3-0 in the round-robin since Garbiñe Muguruza did so in 2015.

Stephens finished the match with 25 winners, seven more than her opponent, and won two-thirds of points on the German's second service. Kerber was also undone by 38 unforced errors throughout the clash, and went only 2-for-10 on break points, including 0-for-7 to start the match.

Stephens completes the semifinal lineup, joining fellow 5-8 seeds Elina Svitolina, Karolina Pliskova, and Kiki Bertens in the final four, as the top four seeds all fell in the round-robin. Red Group winner Stephens will face Karolina Pliskova in the semifinals, as the Czech finished second in the White Group.

"Me, Svitolina, Bertens, and Karolina, I think we have all had great seasons and had some really great results," said Stephens. "I think that obviously the top four have, too, but I think at the end, like when we were all battling towards the end of the year, everyone pretty much counted us out, because they were, like, 'Oh, they are barely going to qualify or barely get in.'"

"I think we have all played, the four of us in our groups, I think we have all played really well," Stephens added. "When I played Kiki, that was probably the best match I played in the group. So, yeah, I think it's great for us, great for the younger girls just to be able to prove ourselves and even get this far and get here."

In the opening stages, Stephens struggled on serve, but stayed level with Kerber through 2-2, withstanding five break points in the process and saving many of her most outrageous winners while down in the games.

That fight would pay dividends when the American blasted an overhead winner to convert her first break point of the match and take a 3-2 lead over Kerber. The German again had chances on a Stephens service game, but Stephens saved two more break points, one with a stunning backhand winner on the sideline, to hold for 4-2.

Now 0-for-7 on break points, Kerber’s frustration leaked into her next service game, which she dropped at love for 5-2. Serving for the set, Stephens finally appeared flustered, squandering three set points, the last with a very makeable forehand that flew long, before Kerber finally converted a break point on her eighth try.

But it would be for naught as Kerber once again faltered on serve. Down 15-30, the top seed pushed a drop shot attempt into the net to hand Stephens two more set points. The American took advantage as soon as she could, firing a deep backhand to force an error from the German and grab a one-set lead.

Stephens and Kerber each had 15 unforced errors in the first set, but Kerber had only seven winners while Stephens fired 17 en route to a commanding first-set triumph.

The combatants continued to blast shots at each other in grueling baseline rallies as the second set got underway, with more protracted games taking place. Stephens used her sturdy backhand to attain a difficult hold at 2-1, then seemed to crack the code for good by breaking Kerber for 3-1 on her staggering seventh break point of a marathon game.

The German initially refused to cede after what appeared to be decisive service break, though, and evaded three game points in the next game to get back on serve, after the American faltered due to surprising unforced errors.

Kerber, however, had a difficult service game at 4-3. The German saved three break points as her forehand briefly caught fire, but Stephens reached a fourth chance after a backhand error by Kerber. This time, the break point went the American’s way, as Kerber punched a forehand miscue into the net. A love hold by Stephens then sealed her semifinal spot.

Stephens will now go up against Pliskova in the final four. The American leads the head-to-head 2-1, but Pliskova won their only match this season, on the clay of Madrid. Stephens's two wins came in 2012 and 2015.

"[Pliskova] has a really big serve, but I think the court kind of neutralizes that," Stephens said. "I think it will be a good match. Obviously playing semis, you're playing, I don't know, what are we, 8 and 9 in the world, something like that?"

"A single-digit number when you're playing someone that's a single-digit number is always tough. I'm just going to go out there and play my best and do what I have been doing all week and try to have another good match."