RIYAD, Saudi Arabia — Heading into Monday’s pivotal second matches at the WTA Finals Riyadh presented by PIF, Elena Rybakina and Zheng Qinwen faced radically different challenges.

Rybakina, beleaguered by insomnia, allergies and injuries, had played only four matches since Wimbledon in mid-July. Zheng, on the other hand, was a victim of her own breakthrough success. Her match total in that same span was a stout 33, including titles in Palermo and Tokyo and an Olympic gold-medal run in Paris, and fatigue had to be a factor.

In this meeting of 0-1 players, which set of taxing circumstances would be easier to overcome? Would fitness be a problem for Rybakina — fatigue for Zheng?

In the end, the No.7-seeded Zheng was a 7-6 (4), 3-6, 6-1 winner. It was the first career win over Rybakina in three tries for the in-form 22-year-old.

With No.1 Aryna Sabalenka’s later 6-3, 7-5 victory over No.4 Jasmine Paolini, Rybakina (0-2) is eliminated from contention. Sabalenka (2-0) is the Purple Group winner, while Wednesday’s Zheng-Paolini winner will also advance to the semifinals.

In addition, if World No.2 Iga Swiatek loses to No.3 Coco Gauff on Tuesday, Sabalenka clinches the year-end No.1 ranking.

Afterward, Zheng lamented the fact that she broke Rybakina first in both the first and second sets — and yet the match went the distance.

“I was the one to make the first break, and then I let her back in the first,” Zheng said. “Then it happened again in second. But in the third set I talked to myself. I broke her first and I stayed focused.

“Finally in the third set I start to play my tennis again. So I’m really happy to win the match.”

This felt like another “home” game for Zheng, as the crowd at King Saud University Arena was solidly behind her. There were eight Chinese flags being waved vigorously during changeovers and supportive chants occasionally broke out.

“Fans yelling my nickname, like I am in China,” Zheng said after her first-round loss to Sabalenka. “Few fans quite crazy, always screaming so hard my name.”

These are two of the most powerful players on the Hologic WTA Tour. Zheng leads all WTA Tour players with 420 aces this year, with Rybakina second at 352. And that power struggle defined the match. They returned aggressively, too, which was probably why the only two breaks in the first set came when the server double faulted.

In the tiebreak, Zheng won both of Rybakina’s first two service points and maintained that margin, taking it when a Rybakina backhand went long.

With Rybakina serving at 3-all in the second set, Zheng forged a pair of break points — which Rybakina erased with back-to-back aces. Her third of the game led to a 4-3 lead that soon became 5-3 after a loose service game from Zheng.

With Rybakina serving at 1-2, Zheng scored the first break of the third set by guessing right on a first serve and lashing a backhand return winner. Several times in that game, Rybakina hit weary looking forehands.

A break in the sixth game sealed it for Zheng.