Casper Ruud will face Daniil Medvedev in the last four of the ATP Finals after coming from behind to beat Andrey Rublev 2-6, 7-5, 7-6 (7/5) on Friday.
Both players knew a win at the Pala Alpitour in Turin would take them into the Saturday's semi-finals and Ruud impressed in coming from behind to make sure he will be taking on the world number two.
The Norwegian has followed up a brilliant year in which he won five tournaments by reaching the semis in his first appearance at the season-ending event.
He did it despite being demolished in the first set by fifth seed Rublev, who then looked certain to be facing his countryman at the weekend after breaking Ruud's serve in game three of the second set.
"It's a great day for me and my career obviously, this year has been special," said Ruud on court.
"I've been on the edge for the past two matches, coming back from a set down... I don't know really how I did it.
"I got some good returns and then the momentum changes, we are both playing with nerves in the end so I guess I can thank my arm I was able to hit an ace on the match point because I was so nervous."
Fifth seed Rublev came into the match unbeaten in his four previous meetings with Ruud but after a strong start fell to a deserved defeat, failing to qualify for the knockout rounds from the Green Group.
"I don't have much to say, it's tough. Hopefully I can get through it to give myself a lesson." a dejected Rublev later told reporters.
The way Rublev strolled through the first set suggested a quick victory but Ruud fought back gamely in the second, forcing his higher-ranked opponent into unforced errors and hitting 13 winners as he broke Rublev twice to take the set.
Rublev was visibly angry with the way his performance levels dropped, taking his frustration out on his racket after allowing Ruud to immediately break back to level the set at 2-2 when he had the match by the scruff of the neck.
From there Ruud gained further confidence and wasn't even fazed when his serve was broken in game five of the third set, hitting straight back at Rublev to level the scores.
He eventually came through the tie-break afer a tense round of games held to serve, in which both players pulled out some beautiful shots, to set up a clash with US Open champion Medvedev which Ruud called the biggest of his career to date.