SINGAPORE - Fresh off eliminating Caroline Wozniacki and reaching the semi-finals of the BNP Paribas WTA Finals Singapore presented by SC Global for the first time, one of Elina Svitolina's first actions was to thank the outgoing champion during her on-court interview.

"She always brings out the best in you and you have to really push yourself, so I want to say 'thank you' to her - she really pushed my limits," said Svitolina, who beat the Dane 5-7, 7-5, 6-3 at the Singapore Indoor Stadium on Thursday (Oct 25).

The world No. 7 had needed to win a set to secure her spot in the last four of the US$7 million (S$9.7 million) tournament, with Karolina Pliskova having qualified earlier in the day.

The 2hr 35min battle saw world No. 3 Wozniacki and Svitolina exchange long rallies, with the former dictating the points better during the first set which took 57 minutes.

With both players on serve in the second set, Svitolina, 24, closed it out on her fifth set point after a Wozniacki backhand sailed long.

The Ukrainian roared in triumph and did not look back in the third set, breaking Wozniacki's serve to lead 5-3 and saving four break points in the final game before sealing the win with a drop shot.

Said Svitolina of those five set points in the second set: "She was playing like a champion and I was expecting this because (in) all the matches before, when she was down, she was going for it.

"That's why when I finally got my fifth set point, I told myself I was going to be as tough as nails and (I was) not going to give anything."

Wozniacki gave credit to Svitolina for taking her chances, and said: "She just played better than me today and you just have to take the loss and that's it."

The 28-year-old, who won her third title of the year at the China Open earlier this month, revealed during her post-match press conference that she was diagnosed with rheumatoid arthritis before the US Open.

She said: "It turns out that I have an autoimmune disease, which is rheumatoid arthritis... It goes in and attacks your joints.

"You just have to move on from it and figure out how to deal with it and live with it... I'm very proud of how I have been so positive through it all and just kind of tried to not let that hinder me."

Svitolina's third win in a row earned her top spot in the White group, ahead of Pliskova. The 26-year-old fought back from a 4-2 deficit in the second set to beat fellow Czech Petra Kvitova 6-3, 6-4.

The other two semi-finalists will be decided on Friday, following the last round of matches in the Red group.