Marin Cilic defeated Novak Djokovic in straight sets to eliminate the Paris Masters champion and leave his status as world number one under severe threat from Andy Murray.
Djokovic, who has struggled badly for consistency since exiting Wimbledon in the third round in July, was beaten 6-4 7-6 (7-2) by 2014 US Open winner Cilic, who defeated the 12-time grand slam singles champion for the first time in 15 attempts.
Should Murray overcome Tomas Berdych in the last eight and then get past Milos Raonic or Jo-Wilfried Tsonga to reach the final of the ATP Masters-level event, he will become Great Britain's first world number one.
Top seed Djokovic had to recover from losing the opening set to Grigor Dimitrov on Thursday and he made a poor start again a day later, sending two forehands long and then netting a routine rally ball to be broken in his first service game from 40-30 ahead.
And despite getting the break back immediately, the Serbian uncharacteristically wilted again in the 10th game of the set, netting an attempt at a cross-court backhand return as Cilic combined his booming serve with some heavy ground strokes to good effect.
Djokovic looked to have taken control of the second set when he broke Cilic to love and earned a 5-4 lead, but the inconsistency that has plagued the 29-year-old in the second half of the season was evident yet again, as a double fault allowed Cilic to break back.
Djokovic was at least able to save two match points to come from 15-40 down and eventually hold to send the contest to a tie-break.
But his scattergun serve proved fatal in the decider. Cilic, as he had done throughout the match, punished a weak second serve to bring up four match points and the Croatian needed only one to complete the upset.