Elina Svitolina subjected Simona Halep to more WTA Finals agony with a straight-sets victory which knocked the world number one out and put Caroline Garcia through to the semi-finals.
Halep knew a win in the final round-robin match would ensure she progressed to the last four, but Svitolina did Garcia a huge favour by beating the top seed 6-3 6-4 in Singapore on Friday.
Fourth seed Svitolina had no chance of advancing after Garcia mounted a stunning fightback to beat Caroline Wozniacki earlier in the day, but the debutant produced easily her best performance of the tournament to claim her first victory.
A frustrated Halep paid the price for too many errors under pressure and was left with a sense of deja vu, having now crashed out of the season-ending event in three consecutive years after losing her final group match.
Svitolina's success means Garcia has come from nowhere to top the Red Group and will face Venus Williams in the last four, with Wozniacki coming up against White Group winner Karolina Pliskova, who is still in the running to clinch top spot in the rankings.
A relaxed Svitolina was playing without pressure and it was Halep who felt the heat, making an early call for a visit from her coach Darren Cahill after losing the first three games.
An aggressive Svitolina was getting everything back and there were too many unforced errors from Halep, who grew in confidence but was unable to find a way to break the vibrant Ukrainian.
There was a double fault from Svitolina on set point, but she responded with a vicious serve to earn another set point and this time made no mistake as Halep could only drill a backhand return into the net.
Halep showed great speed around the court and reaped the rewards of a more positive approach in Svitolina's opening service game of the second set, fighting back from 0-40 down to earn an elusive first break.
That advantage was short-lived, though, as Svitolina broke straight back and levelled at 2-2 with a hold to love. Svitolina had the momentum and was scenting victory after breaking again to lead 4-3, a seething Halep's frustration growing as she sat with a towel over her head at the changeover.
The outstanding Svitolina was enjoying herself, turning defence into attack to win a sublime rally which almost raised the roof and sealing her maiden WTA Finals win when an off colour Halep netted to bring her season of ups and downs to an anticlimactic end.
Svitolina  bt Halep  6-3 6-4
Svitolina - 14/15
Halep - 22/19
Svitolina - 1/1
Halep - 2/1
BREAK POINTS WON
Svitolina - 3/5
Halep - 1/1
FIRST SERVE PERCENTAGE
Svitolina - 67
Halep - 72
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Svitolina - 69/43
Halep - 71/68
Svitolina - 59
Halep - 48