Bianca Andreescu failed to shake off a knee injury as she fell to a third straight defeat and departed the WTA Finals, where defending champion Elina Svitolina advanced from the Purple Group.
US Open champion Andreescu, still just 19, had been enjoying an incredible 17-match winning run on the WTA Tour until coming up short in a gripping encounter against Naomi Osaka at the China Open.
The Canadian did not play again until this week's year-end Finals, making a losing return in a narrow defeat to idol Simona Halep.
And any hopes of Andreescu arresting that mini-slump were ended by an issue with her left knee in Shenzhen as she bowed out to world number two Karolina Pliskova.
Andreescu, needing to win to stay in the tournament, broke in the first game of the match but was soon limping, retreating to her seat and calling a medical timeout.
Heavy strapping was applied to the teenager's knee, yet she continued to move uneasily as she sought to continue, seeing Pliskova break back immediately.
Andreescu told Sylvain Bruneau she "heard a crack" and was struggling to bend her knee but insisted she was still capable of beating Pliskova despite the advice of her coach.
"I don't want to stop," she said. "If it was against Simona, then okay. But [Pliskova] misses."
Andreescu showed grit to then hold after saving three break points, but it was only a temporary reprieve as Pliskova sent down a backhand return to seize the advantage in her opponent's next service game.
Pliskova held to take the set 6-3 and Andreescu finally gave in, retiring for a defeat that sees her out of the competition as the Czech faces Halep for a place in the last four.
Halep had wasted an opportunity to book her spot in the semi-finals early on Wednesday when she was beaten 7-5 6-3 by Svitolina.
The Wimbledon champion battled back from 3-0 down early on but then missed the chance to break in front with a poor call on a drop shot, subsequently losing the opener with an error-strewn final service game.
Having been behind throughout the first set, Halep led in the second with a break to love but was quickly pegged back, soon complaining to coach Darren Cahill that she felt unable to disrupt Svitolina's game.
Svitolina, who defeated Sloane Stephens in the 2018 final, preyed on more Halep mistakes to break in front and wrapped up a relatively comfortable triumph in straight sets.