US Open finalist Pegula, 30, had got off to the perfect start with a love hold in her first service game on Tuesday, amid a catalogue of Krejcikova unforced errors.

But it was Czech Krejcikova, who is ranked 13th in the world but qualified for the WTA Finals on account of being a 2024 Grand Slam champion, who got the initial break midway through the first set, at which point sixth seed Pegula's body language began to sour.

Krejcikova missed a chance to wrap up the opener on her first set point as she slammed a backhand return into the net, but she was gifted it on her second as Pegula tapped a return into the net from close range.

Form did not improve for Pegula at the start of the second set as Krejcikova broke her serve at the first opportunity, though there was a brief reprieve for the American as she cancelled that out in the next game.

But any glimmer of hope for a Pegula comeback was extinguished as she double-faulted to give her opponent the break back, after which Krejcikova coasted to a straightforward victory with a solitary break point her only hurdle.

Though Krejcikova's performance was from far from polished, finishing with more unforced errors than her opponent, her serving did much of the damage as she recorded 11 aces - her highest total of the season - compared to one from Pegula.

Pegula will face Swiatek in her final group-stage match on Thursday, after Gauff plays Krejcikova.

The WTA Finals are being held in Saudi Arabia for the first time - a move which has been criticised by some because of the country's human rights record.

This year's tournament has record prize money, with the singles champion set to collect about 4m.