FORT WORTH, Texas (AP)Maria Sakkari emerged to take a pair of tiebreakers and edge Jessica Pegula across more than two hours of big-hitting baseline action in front of a sparse crowd Monday as round-robin singles play began at the season-ending WTA Finals.

Playing with her grandmother in the stands, the No. 5-ranked Sakkari’s 7-6 (6), 7-6 (4) victory on a temporary indoor hard court at Dickies Arena came in a rematch of the Guadalajara Open final on Oct. 23.

Pegula won that one 6-2, 6-3 to claim the first WTA 1000 title of her career and move up to No. 3.

”It’s never easy against Jess,” said Sakkari, who was a two-time Grand Slam semifinalist last season.

The other matches scheduled for later on Day 1 were Ons Jabeur vs. Aryna Sabalenka in singles, and Pegula and Coco Gauff against Xu Yifan and Yang Zhaoxuan in doubles.

Sakkari vs. Pegula was filled with plenty of momentum swings.

Sakkari won the first two games. Pegula took the next three. Sakkari broke to lead 6-5 and serve for the first set. Pegula broke right back when Sakkari flubbed a drop shot, then chucked a ball and walked over to a towel holder and gave it a kick.

In the tiebreaker, Sakkari finally closed it out on her third set point with a forehand passing shot, 1 hour and 11 minutes after the contest started.

The second set followed a similar pattern: Less than 15 minutes in, Sakkari led 3-0, but soon enough it was 3-all.

Sakkari had an opportunity to close out her first win against a top-five opponent this year when she held two match points at 6-5 in second. But she dropped a backhand into the net on the first, and Pegula delivered a backhand winner on the second.

Then, in the closing tiebreaker, Sakkari converted her third match point on a forehand into the net by Pegula, who had won 11 of her preceding 13 matches – with both losses to No. 1-ranked Iga Swiatek.

Being held in the United States for the first time since 2005, the WTA Finals began with a bit of a hiccup – an odd delay of several minutes caused by a questioned line call on the third point – and a victory for defending doubles champions Barbora Krejcikova and Katerina Siniakova.

The Czech duo, who won three major championships together this season and completed a career Grand Slam, beat Desirae Krawczyk of the U.S. and Demi Schuurs of Netherlands 6-4, 6-3.

There were a few hundred spectators scattered around the stands at the outset, leaving thousands of empty gray seats, and the courtside radar readouts initially displayed serve speeds in kilometers per hour instead of the more U.S.-friendly miles per hour, which was quickly changed.

With Krawczyk serving to Krejcikova at 30-love in the very first game, the electronic line-calling system ruled a 100 mph offering an ace. Krejcikova held a finger and thumb apart to show that she thought the ball landed out and asked to see a replay. No such video ever appeared.

That prompted a lengthy break as players and the chair umpire discussed how to handle the situation. Eventually, the point was replayed – and again won by Krawczyk, this time with a passing shot.

AP tennis: https://apnews.com/hub/tennis and https://twitter.com/AP-Sports