Andy Murray must overcome Milos Raonic to become world number one after Novak Djokovic's quarter-final exit from the Paris Masters on Friday.
Marin Cilic won 6-4 7-6 (7-2) to oust the defending champion from the ATP Masters event, meaning the Serbian's ranking destiny is now out of his hands.
Following Djokovic's loss, and with the knowledge he was just two victories away from a long-awaited coronation, Murray duly saw off Tomas Berdych in straight sets to advance to the semi-finals.
After a tight first set that stayed on serve, Murray demonstrated impressive resilience to recover from 6-1 down in the tie-break and win it 11-9.
He then survived being broken while serving for the match to break back and eventually serve it out, securing a 7-6 (11-9) 7-5 victory.
Raonic, who proved no match for Murray in the final of Wimbledon earlier this year, is the last obstacle in his path after the Canadian defeated Jo-Wilfried Tsonga 6-2 7-6 (7-4) to reach the last four.
Tsonga's defeat, which came after the entertaining Frenchman gave up a crucial mini-break in front of his home fans in a second set tie-break, saw Austrian Dominic Thiem qualify for the ATP World Tour Finals in London later this month.
Earlier, the inconsistency that has plagued Djokovic since his early exit from the All England Club in July was present again as the Serbian gave up two easy breaks to fall behind to Cilic, who used his own booming serve in good combination with heavy ground strokes to punish his out-of-sorts opponent.
Djokovic had a chance to level the match when he broke the Croatian to serve for the second set at 5-4, only to be broken again.
The 12-time grand slam singles champion demonstrated flashes of his famous determination to save two match points and force a tie-break, but his serve again let him down in the decider, Cilic winning it on the first of four more match points to beat his nemesis for the first time in 15 career attempts.
In the opening match of the day, John Isner battled past fellow American Jock Sock to win 7-6 (8-6) 4-6 6-4.
Sock saved the only two break points in a tight opener but came out worse off in the tie-break and, while Isner was broken twice in succession to lose the second set, it was Sock who faltered early in the decisive third.
And the 24-year-old was unable to recover from giving up his serve in the first and third games, Isner advancing to a showdown with another big-serving giant in the form of Cilic.