The Russian, who fell behind 4‑1 in the first set, rallied to win the next 11 games, sealing the round‑robin victory in just 62 minutes.

“Oh my God, I’m even happier than I was after my first match,” Andreeva said, adding that she felt full of energy and ready for two more matches.

Andreeva, the 19‑year‑old world number three and French Open champion, is making her singles debut at the season‑ending event after already securing three titles this year.

Sabalenka, the second‑seed and world number two, struggled early after a double fault and a 0‑40 break point, later saving three break points before the Russian broke again to go up 3‑0 in the second set.

Elina Svitolina kept her own bid on track with a three‑set win over Karolina Muchova, ensuring the Group 2 contest remained lively.

The win not only makes Andreeva the Group 2 winner but also keeps the battle for the year‑end No. 2 ranking very much alive.

Brief scores: Mirra Andreeva 6‑4, 6‑0 Aryna Sabalenka – Andreeva advances to the semifinals of the WTA Finals.