Paris - Andy Murray moved to within one victory of becoming the world's top-ranked player for the first time after defeating Tomas Berdych in the Paris Masters quarter-finals on Friday.
Marin Cilic had earlier dumped out a struggling Novak Djokovic to pave the way for Murray to supplant the Serb at the top.
But Murray was forced to save seven set points in an extraordinary opening tie-break before clinching a 7-6 (11/9), 7-5 win in just under two hours.
The Briton will meet Milos Raonic or Jo-Wilfried Tsonga in the semi-finals knowing that a victory will move him above Djokovic, who has been on top of the world rankings for 122 weeks.
"Maybe there are a few more nerves today knowing that the world number one was possible but once you are out there on court it didn't bother me," Murray told Sky Sports.
"I have never been in that position of playing for the world number one spot before so I don't know how I will deal with it tomorrow."
Neither Murray nor Berdych showed any signs of fragility as the first set went on serve, but the wheels fell off in spectacular fashion for the Czech after he raced 6-1 ahead in the tie-break.
Murray chalked up six straight points to earn a set point of his own, with Berdych then squandering two more before the second seed finally edged ahead.
He broke Berdych to start the second set and looked on course to wrap things up, but a lapse at 5-4 saw him require a second chance.
Berdych, who needed to reach the last four to stay in the race for the Tour Finals in London, handed him that opportunity by again losing his serve, with Murray sealing it on his fourth match point.
Cilic had lost all 14 previous meetings with Djokovic but snapped that miserable run with a 6-4, 7-6 (7/2) victory.
Djokovic, the record four-time champion, 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.
"Credit to Marin and congratulations. He definitely played better today, and he deserved to win," said 29-year-old Djokovic.
"I wasn't on the level that I could have been on. Obviously, there are things I could have done better. And just in important moments I wasn't able to deliver."
Ninth seed Cilic, who on Thursday became the seventh London qualifier, 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.
The top seed uncharacteristically gifted the break back, though, after a pair of costly double faults, and Cilic had him on the ropes as two match points passed by at 6-5 on a faltering Djokovic serve.
There was to be no remarkable recovery either 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," said Cilic.
"I felt on the court really, really good, and it's a great time for me. As I was mentioning these last few days, the end of the season is just getting better and better."
American John Isner awaits Cilic in last four after he outlasted compatriot Jack Sock 7-6, (8/6), 4-6, 6-4.
Isner and Sock teamed up to win the doubles in Shanghai last month, but a 33rd match -- singles and doubles combined -- in as many days for Sock ultimately proved a step too far.