A sensational performance from Daniil Medvedev saw Rafael Nadal's long wait for a triumph at the ATP Finals continue in London.
Nadal, who clinched his 13th Roland Garros crown and wanted to end his year with a first win at the season-ending event, won the opening set and looked in fine shape.
But the fourth seed was not to be counted out and continued his unbeaten run at the O2 Arena by storming back in fine fashion to prevail.
"To beat Rafa you need everything" Medvedev after overcoming Nadal to reach final
The crucial moment in the match came in a tense and dramatic second set tie-break, which Medvedev eventually came through to win 7-4.
The Russian had too much for Nadal in the decisive third set and simply overpowered his opponent in a very impressive display.
Medvedev will now take on US Open champion Dominic Thiem in a hotly-anticipated final on Sunday with a few hours' rest.
Rafael Nadal after Medvedev defeat: I lost a big opportunity
Thiem leads the pair's head-to-head 3-1 and will have enjoyed a bit more recovery than the 24-year-old.
However, Medvedev has been in sublime form the last few weeks and could now follow his triumph at the Paris Masters with an even bigger title in London.
Sunday's final will be the last ATP Finals match played in London's O2 Arena before the event switches to Turin in 2021.
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