Daniil Medvedev fought back to beat Rafael Nadal and set up a title decider with Dominic Thiem at the ATP Finals in London.
Medvedev battled back from a set down to win 3-6 7-6 (7-4) 6-3 in London - his first victory over the 20-times Grand Slam champion.
The 24-year-old Medvedev reached the semi-finals as the only player to post a 100 percent record in his group, including a superb defeat of Djokovic.
He was the better player in the opening set against Nadal but one lapse on serve was all the Spaniard required to snatch it.
Medvedev blew a big lead in the second set when Nadal was one point away from a going down 5-1. Nadal swung the match his way and served for a place in the final at 5-4, only for Medvedev to break to love.
A flukey forehand winner helped Medvedev move 5-3 ahead in the tiebreaker and he took full advantage to set up a decider.
Nadal appeared to be feeling the pace in a tense final set and some sensational winners allowed Medvedev to grab the crucial break of serve in the seventh game.
Earlier, Thiem came back from the brink of defeat to beat Djokovic in a gripping last-four clash, as he reached the final for the second successive year.
The 27-year-old Austrian appeared to be sliding to defeat when he trailed 0-4 in the third-set tiebreak but stormed back to claim a 7-5 6-7(10) 7-6(5) victory in just under three hours.
World No. 3 Thiem was the better player in the first set, sealing it with an ace after breaking serve at 5-5.
An absorbing second-set tiebreak saw Thiem get four chances to seal victory but he double-faulted on one match point and Djokovic showed true grit to snatch the set.
Serves dominated in the decider and when Thiem started the day's second tiebreaker and then lost the next three points it seemed as though the end was nigh.
Instead the world number three roared back, winning the next six points.
Djokovic saved a fifth match point but Thiem finally got the job done when a Djokovic groundstroke drifted long.
Thiem will now try to complete a remarkable year, in which he claimed his first Grand Slam title by winning the US Open, against either Rafael Nadal or Daniil Medvedev in Monday's climax.
"It was a mental battle, I got so tight in the second-set tiebreak,"says Thiem
"To play these legends is always special and to player here is also something special. I thought after winning the title in New York I would be more calm, but that was a mistake I guess."
Thiem will attempt to go one better than last year when he lost to Stefanos Tsitsipas in the final.
Defeat means Djokovic will have to wait until the event moves to Turin next year, after 12 years in London, to try again to equal Roger Federer's six titles at the year-ender.