Andy Murray needs to only win his semi-final against Canadian Milos Raonic to bring to an end the Serb's reign as number one that started in July 2014.
Novak Djokovic was dumped out of the Paris Masters in straight sets by Marin Cilic on Friday, opening the door for Andy Murray to end the Serb's two-year stranglehold on the world number one ranking.
Djokovic, chasing a fifth title in Paris, slumped to his first defeat by big-serving Croat Cilic in 15 matches as he went down 6-4, 7-6(2).
The defeat means Murray, who ousted Czech Tomas Berdych 7-6(9), 7-5, needs to only win his semi-final against Canadian Milos Raonic to bring to an end the Serb's reign as number one that started in July 2014.
"He definitely played better today and deserved to win. I wasn't at the level I could have been," said Djokovic.
"I was in a good position to take the match into the third set but hit two double faults. In important moments I wasn't able to deliver."
Djokovic, who needed to reach the final to guarantee keeping Murray at bay, saved two match points at 6-5 in the second set, the latter with a superb forehand cross court, but was soon facing a losing battle in the tiebreak.
Cilic, who on Thursday clinched his spot in the World Tour Finals in London later this month, raced into a 5-2 lead before a huge serve set up four more match points.
He was soon celebrating after swiping away a mid-court forehand to claim victory in one hour 45 minutes.
Asked about Murray replacing him as number one, 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 -- much respect for what he has done."
Murray came through an exciting encounter with seventh seed Berdych.
The Scot saved five set points, and seven in all, after going 6-1 down in a captivating 16-minute tiebreak that he eventually took 11-9 with a forehand crosscourt winner.
Berdych, realising he had missed a golden opportunity, had his serve broken in the opening game of the second set.
The Czech broke back when his opponent served for the match at 5-4 but Murray responded by breaking again before he completed victory with a venomous ace down the middle.
"Maybe a little bit before the match there were a few more nerves but once I got out there I didn't let it affect me," said the Scot. "It was a great atmosphere and a packed crowd.
"My goal wasn't to get to number one this year, I was looking at the beginning of next year as the chance to do it. If it happens this week then great."
Fourth seed Raonic progressed with a 6-2 7-6 (4) victory over Frenchman Jo-Wilfried Tsonga.
Heavy-serving John Isner, who is unseeded, secured a meeting with Cilic after firing 17 aces as he beat fellow American Jack Sock 7-6(6) 4-6 6-4.