The Belarusian struggled to maintain her composure throughout the encounter, with her frustration becoming increasingly visible as the match progressed. Sabalenka threw her racquet to the ground and crouched at the back of the court after losing crucial points, eventually watching her hopes of reaching the last four disappear.
After losing control towards the end of the opening set, the former world number one managed to recover and force a deciding set. However, her inconsistent performance returned, allowing Svitolina to dominate the final stages and win the last six games of the contest.
The defeat came just two days after Sabalenka suffered another disappointing loss against Russian teenager Mirra Andreeva, during which she dropped 11 consecutive games.
Svitolina, who won the WTA Finals in 2018, will now face Czech player Linda Noskova in the semi-finals. The Ukrainian secured second place in her group behind Andreeva, who will meet Jessica Pegula in the other semi-final.
Speaking after her victory, Svitolina described the encounter as mentally challenging, explaining that she concentrated on taking each point individually rather than becoming distracted by the pressure.
Meanwhile, Sabalenka will reflect on a season that started with considerable promise but ended without the major title she had hoped to secure. The 28-year-old reached the finals of both the Australian Open and US Open but failed to win either tournament.
She did, however, collect three other titles, including victories at Indian Wells and Miami to complete the prestigious Sunshine Double. Sabalenka also recorded 47 victories from 58 matches, giving her the highest winning percentage on the women's tour this season.
Despite her achievements, she lost the world number one ranking to Elena Rybakina after their US Open final encounter. Sabalenka had held the position for 99 consecutive weeks.
Elsewhere, Andreeva continued her impressive debut campaign by defeating compatriot Diana Shnaider 6-2 6-1 to complete the group stage unbeaten. The 19-year-old became the youngest player in 17 years to reach the tournament's knockout stages.
