World number six Elina Svitolina believes a stronger mental resolve is the backbone behind her impressive run at the WTA Finals, as the Ukrainian eyes a season-ending triumph in Singapore.

The 24-year-old qualified for the semi-finals after overcoming defending champion Caroline Wozniacki 5-7, 7-5, 6-3 in two hours and 35 minutes on Thursday to finish top of White Group.

Svitolina had an early exit in her WTA Finals debut last year but has looked a different player this time around after also defeating Czech players Petra Kvitova and Karolina Pliskova earlier in the tournament.

It has been an unexpected run after a turbulent period, losing in the first round of Wimbledon and splitting up with coach Thierry Ascione. She also had to deal with criticism after changing her physique and becoming leaner.

Svitolina believed the adversity had made her stronger.

“I had a really, really tough second part of the season,” she told reporters. “With all the things going on social media... with the opinions about my physique, it wasn’t easy.

“For me, going into this tournament was very important mentally to bounce back. So this (WTA Finals) really shows that I’m getting mentally tough.”

Svitolina has been unable to get past the quarter-finals of a Grand Slam, fuelling doubts over her standing on the big stage.

But in Singapore, she has dug deep and impressively fought back from a set down against Wozniacki to book a semi-final berth.

“The job is not done yet but these three matches that I had, they have been really tough,” she said. “Today I saw off this challenge.

“I’m really, really happy the way I could handle it.”

Pliskova joined Svitolina as White Group qualifiers, while the remaining semi-spots will be determined on Friday in the final round-robin matches in Red Group.