SINGAPORE - No.5 seed Sloane Stephens held off a determined Kiki Bertens in a late-night thriller at the BNP Paribas WTA Finals Singapore presented by SC Global - ensuring that the Red Group will remain wide open until the last day of round robin play.

Both players came into the encounter with a chance to top the Red Group and advance into the semifinals: Stephens would have sealed the berth with a straight sets win, while Bertens - thanks to Angelique Kerber’s three-set win over Naomi Osaka - simply needed a victory.

Stephens ensured that neither of those scenarios came to pass, dropping a set en route to recording her 7-6(4), 2-6, 6-3 the victory clocking in at nearly 1 a.m. in Singapore.

Read more: WTA Finals 2018 Live Blog: Stephens survives Bertens onslaught

“I think we are kind of similar in that we can mix it up a lot and do a lot of different things,” Stephens said of her opponent. “So I thought that's why we had so many great points, long points where there were slices and lobs and all types of different things going on.

“But I think that's why it was so competitive, because we were playing kind of similar. But it was a tough match. I think it's not often you play someone who plays kind of similar to you that you have to kind of battle it out and figure it out.

“It was kind of cool, but I thought we played some really great points. I guess that's kind of what competing is all about.”

Stephens made a fast start against Bertens, playing nearly untouchable tennis to bring up a 3-0 lead. But Bertens - who owned a streak of nine consecutive Top 10 wins coming into the match - quickly turned things around and got them back on serve at 3-2.

The pair stayed deadlocked - trading breaks then serving out the set - to send them into the first tiebreak of the 2018 WTA Finals. Stephens found another gear to smother her opponent, winning five of the first six points in the decider and edged through to take the opening set.

But Bertens turned things around in the second, going for more of her shots and starting out the set with a 2-0 lead. She extended her advantage to 5-1 before Stephens halted her momentum, saving four set points to extend her stay in the set. But the American couldn’t recover the second break, firing unforced errors as Bertens took the set to send the match into a decider.

Bertens continued her strong momentum, breaking to start with a 2-0 lead in the third set - the sixth of the tournament, marking the most three-set matches at a WTA Finals since 2009. But that’s when it all started to unravel for the Dutch player - Stephens raised her level, capitalizing on Bertens’ growing unforced error count. She reeled off four games in a row to get in front 4-2.

Stephens had an answer for everything that Bertens sent her way, outlasting the Dutch player across the next three games and notching a final break to love to seal the victory after a grueling two hours and 22 minutes.

Stephens fired 18 winners across three sets - offset by 37 unforced errors - against Bertens’ 24 and 47, respectively. They both converted the same amount of break chances, with Stephens winning seven from 13 opportunities and Bertens seven from nine - but Stephens rose to the occasion on the big moments to claim the victory.

Her win sends a big ripple effect across the Red Group - and no one is more relieved than Osaka. As it stands, all four players are in contention for the two semifinal spots, with Stephens topping the Red Group with two wins and no losses (4-2 in sets), and Kerber in the No.2 spot followed by Bertens - with both players tied in wins and sets (1-1, and 3-3, respectively) Kerber claims the advantage in games, leading 30-28 against Bertens’ 28-28. Osaka rounds out the group in the No.4 spot with a 0-2 record - a Bertens win today would have seen her be eliminated from contention.  

Read more: 2018 WTA Finals Qualification Scenarios: All to play for in final round of group play

Up next, Stephens takes on top seed Angelique Kerber on Friday, the last day of Red Group play. She needs to win in two or three sets to ensure her passage into the semifinals.

“You don't play like back-to-back Top 10 players like in regular tournaments,” Stephens said. “I mean, you can, obviously, but it's very rare. Like the way seeding works and blah, blah, blah, unless you're in the semis or finals.

“It's kind of cool, actually… it won't happen again for a whole other year, but it's kind of cool to just play against the best. Like every match is like a final. Yeah. Pretty interesting.”