Andy Murray will become world No 1 if he reaches the Paris Masters final after Novak Djokovic crashed out in the quarter-finals.
Djokovic, who had been hunting a fifth title in Paris, lost 4-6 6-7 to Marin Cilic to open the door for current No 2 Murray to become the world's top-ranked player for the first time.
The Serb, who has topped the rankings for 122 consecutive weeks, had never previously lost to Cilic but meekly surrendered that perfect record in straight sets.
Asked about Murray possibly overtaking him as No 1, Djokovic said: "He's definitely a player who deserves that. To see how he has raised his level in the last 12 months is quite extraordinary. We have known each other since very, very early days.
"We were, I think, 11 years old when we first played against each other. Undoubtedly, much respect for what he has done."
Cilic, in buoyant mood after clinching his spot in the World Tour Finals on Thursday, seized his second set-point chance in the opener to take the initiative and put the pressure firmly on Djokovic.
Cilic missed two break-point chances for a 2-0 lead in the second set and looked like he could regret his missed golden opportunity when his opponent broke to serve to get back to parity at 5-5.
But the Serbian produced two double faults as he allowed Cilic to break back and moved to the brink of defeat again when Cilic fashioned two match points at 6-5.
Djokovic saved both of them as he forced a tie-break, but the Croatian wouldn't be denied and pushed on to battle into the semi-finals after swiping away a mid-court forehand to secure a famous win in one hour 45 minutes.
It means Murray, who beat Tomas Berdych in straight sets in his quarter-final, needs only to win one more match in the French capital to end Djokovic's reign at the summit that stretches back to July 2014.
The Scot now just needs to beat Jo-Wilfried Tsonga or Milos Raonic to be the second-oldest player to debut at No 1, after the 30-year-old John Newcombe in 1974.