Novak Djokovic was knocked out of the Paris Masters quarter-finals by Marin Cilic on Friday, leaving the route clear for Andy Murray to become the new world number one if he reaches the final.
Djokovic 122 weeks at number one
Cilic had lost all 14 previous meetings with Djokovic but cast the Serb's 122-week reign at the top of the rankings into serious doubt with a 6-4, 7-6 (7/2) victory.
Murray plays Tomas Berdych for a place in the semi-finals and is just two wins from supplanting long-time rival Djokovic at the summit.
If the Briton defeats Berdych he would then take on Milos Raonic or Jo-Wilfried Tsonga for a chance to claim the number one ranking.
Record four-time champion Djokovic arrived in Paris needing to reach the final to ensure he remained ahead of a rapidly approaching Murray.
But the Serb's bid to register a fourth straight title in the French capital ended abruptly as an improbable escape act fizzled out against an in-form Cilic.
The ninth seed, who on Thursday qualified for the Tour finals in London, broke immediately to signal his intent against a player who had totally dominated him in his career.
In typical fashion Djokovic then hit back straight away to level, but Cilic's confidence was undimmed and the Croat capitalised on a shaky service game from his opponent at 5-4 to grab the opening set.
Former US Open champion Cilic threatened again early in the second, but Djokovic landed a crucial breakthrough at 4-all to serve for the set.
An undeterred Cilic broke back, though, and had the top seed on the ropes as two match points passed him by at 6-5 on the Djokovic serve.
However, there was to be no remarkable recovery this time for Djokovic, as Cilic took charge in the tie-break and closed out a famous win at the fourth attempt.
"It's good for me, I played great tennis. He had beat me 14 times but we had close matches and that helped me play better," Cilic told Sky Sports.