Look at him now.

As the eighth and final qualifier for the ATP Finals, the 22-year-old Norwegian has joined top-ranked Novak Djokovic, No.2 Daniil Medvedev and No.3 Alexander Zverev in the semi-finals of the elite season-ending event.

And he's done so in style.

Ruud showed off aggressive tactics in the decisive tie-breaker and served an ace on his second match point in a 2-6 7-5 7-6 (7-5) victory over Andrey Rublev in his final round-robin match on Friday to secure his unexpected spot in the last four.

Rublev had looked poised for victory in a gruelling match that lasted two hours and 26 minutes when he broke early in the second set having swept the first - his ninth straight set against Rudd.

But Ruud fought back with two breaks of his own to force a decider in which he again bounced back from being broken.

The fifth-ranked Rublev had beaten Ruud in all four of their previous matches.

"I can thank my arm that I was able to hit an ace on match point, because I was so nervous," Ruud said.

Ruud will next face Medvedev, the US Open champion and the defending champion of this event, which is being played in Turin for the first time after 12 years in London.

Djokovic and Zverev will renew their budding rivalry in Saturday's other semi-final.

"Even though I'm the fourth guy I don't consider myself the fourth-best player in the world," Ruud said. "Tomorrow will be a fun day - probably the biggest match of my career."

This is the first time since 1994 that three of the four semi-finalists are former champions - with Ruud the exception.

The round-robin play concluded with Djokovic beating British alternate Cameron Norrie 6-2 6-1 in a mostly meaningless match. Norrie replaced the injured Stefanos Tsitsipas, who is ranked No. 4.

"I played well tonight from the first to the last point," Djokovic said. "That's definitely encouraging for the next match tomorrow which will be tougher."

Australian Associated Press