Aryna Sabalenka opened her campaign for a first WTA Finals title with a hard-fought victory over Wimbledon finalist Karolina Muchova.

In the absence of world No 1 and reigning champion Elena Rybakina, who withdrew on the eve of the tournament with a viral illness, two-time runner-up Sabalenka is the favourite to lift the trophy.

She won the WTA 1000 tournament at the same venue, California’s Indian Wells, in March, and is making her sixth straight appearance at the season-ending event.

The Belarusian was not at her best in a gritty 7-6(2) 7-5 win over Muchova, fighting back from a break down in both sets and producing seven double faults to four aces, only landing 59% of her first serve - her biggest weapon, when it is firing.

She told Sky Sports afterwards: “It was a bit tricky with the sun [and] with the shade. Sometimes I had to adjust my toss, sometimes I had to serve blind, but it's all part of the game.”

Muchova was the only player at the WTA Finals to begin with a winning head-to-head record against Sabalenka, but she twice failed to capitalise on going a break up in the opening set.

Sabalenka fought back and forced a tiebreak after Muchova saved two set points, where she overpowered the Czech and was helped by two big forehand errors from her opponent.

There were another five breaks of serve in the second set but Sabalenka ultimately pulled away. Despite failing to serve out for the match first time around, she made no mistake at the second attempt.

Sabalenka has not won a title since claiming the ‘Sunshine Double’ of WTA 1000 tournaments in Indian Wells and Miami in March.

Later on the second day of the tournament French Open champion Mirra Andreeva was significantly more convincing as she swept past Elina Svitolina 6-4 6-0 in little over an hour.

The Russian broke in the fifth game of the opening set and raced away with the second, saving four break points in the final game as the Ukrainian threatened a comeback.

Andreeva will face Sabalenka in her next match, with the pair now in pole position to progress from the round-robin stage to the semi-finals. The top two in each of the two groups go through.