World number one Aryna Sabalenka entered "beast mode" to defeat Jessica Pegula and close in on a place in the knockout stages of the WTA Finals - after Coco Gauff also won to keep alive her hopes of defending her title.

US Open champion Sabalenka secured back-to-back wins in Riyadh by overcoming American fifth seed Pegula 6-4, 2-6, 6-3, but required a straight-set win to confirm progression from the Stefanie Graf Group.

Instead, Sabalenka will face world number three Gauff in the final round on Thursday, with both players' semi-final ambitions in the balance.

"[Pegula] played incredible tennis. In the second set, she started playing on another level, and when I went a break down in the third, I just told myself I had to play more aggressively," Sabalenka told Sky Sports.

"I felt sorry, but at the same time I was so happy that I was able to find that beast mode inside of me and go after my shots and play unbelievably."

Earlier, American Gauff ensured she remains in contention by eliminating Italy's Jasmine Paolini in an impressive 6-3, 6-2 win.

Having recorded 17 double faults and 75 unforced errors in her opening loss to Pegula on Sunday, the 21-year-old demonstrated her title credentials in an assured performance.

The leading eight women's players in singles and doubles are competing in Saudi Arabia, with the tournament having £12m ($15.5m) in prize money.

Elena Rybakina became the first player to confirm her place in the knockout stages by defeating world number two Iga Swiatek in the Serena Williams Group on Monday.

'Frustrated and stressed' - but Sabalenka prevails

In a competitive, high-quality match, Sabalenka ended Pegula's early resistance with a fourth break point for a 4-2 lead in the first set.

The Belarusian then overcame the frustration of being broken back after missing a first set point by eventually taking her fifth opportunity to close out the set on her opponent's serve.

But Pegula, aware victory by any result would send her through to the semi-finals, took charge of the second set, backing up a break to love by saving two break points against her for a 3-1 lead.

After Sabalenka conceded another break with a double fault - losing a third service game in five - Pegula battled back from two break points down to level the match.

An unsettled Sabalenka was punished for missing her first opportunity of the final set when Pegula clinched her fourth break point to edge in front, but the four-time major winner struck back in an enthralling decider.

That proved crucial as Sabalenka embarked on a four-game streak and, unlike at the Wuhan Open in October, would not allow Pegula to pull off a late fightback - sealing victory with a superb forehand winner.

Sabalenka's coach, Anton Dubrov, left his seat midway through the match after his player became "so frustrated".

"Maybe he did the right move because he really upset me and that's why I was able to pull out something else because I was so stressed," said Sabalenka.

'Definitely a turnaround' - Gauff resets to stay in Finals

French Open champion Gauff reset admirably following her scrappy display against Pegula and reasserted herself as a title challenger with this solid performance.

She produced just three double faults and reduced her total unforced errors by over half (30), with her more consistent serving in particular providing a significant confidence boost before her meeting with Sabalenka.

Calm and confident from the back of the court, Gauff appeared largely in control after taking charge of the opening set.

After breaking world number eight Paolini to love at the first opportunity, Gauff withstood three break points to establish a commanding 3-0 lead - and struck back immediately when the Italian offered a response, before serving out the set.

With 29-year-old Paolini's resistance fading, Gauff converted the first break point of the second set following a superb forehand winner, before sealing victory with a dominant five-game streak.

"I'm really happy with how I played today, it was definitely a turnaround from my first match," said Gauff.

"It's the beauty of this tournament that you have another chance to prove yourself."

In the doubles, second seeds Taylor Townsend and Katarina Siniakova progressed as winners of the Liezel Huber Group.

The American-Czech pair defeated third seeds Erin Routliffe of New Zealand and Canada's Gabriela Dabrowski 6-4 7-6 (7-3).

Their qualification for the semi-finals was later secured when Brazil's Timea Babos and Hungarian Luisa Stefani beat Russians Mirra Andreeva and Diana Shnaider 7-5 2-6 10-7.

That defeat eliminated Andreeva and Shnaider.