Casper Ruud reached the US Open final and closed in on the world number one ranking with a four-set victory over Karen Khachanov on Friday.

Ruud defeated the Russian 7-6 (7/5), 6-2, 5-7, 6-2 in his semi-final and will face either Spanish teenager Carlos Alcaraz or Frances Tiafoe of the United States in Sunday's championship match.

It will be world number seven Ruud's second Grand Slam final of the season after finishing runner-up to Rafael Nadal at the French Open in June.

Should Alcaraz lose his semi-final later Friday, the 23-year-old from Norway will become the new world number one next week.

"This match today was another great match from my side. I think we were both a bit nervous in the beginning, some breaks back and forth," said Ruud who had never got past the third round in New York before this year.

"You have to take into account that this match was probably the biggest match for both of our careers and of course there will always be some nerves."

Ruud and Khachanov, who knocked out Nick Kyrgios in a five-set quarter-final, exchanged a double break each in a scrappy opening set.

However, it ended with a lung-busting flourish when Ruud came out on top in a 55-shot rally to convert a third set point in the tiebreak.

The Norwegian raced to a double break in the third and fifth games of the second set, wrapping it up in just 33 minutes when Khachanov fired a forehand long.

Olympic silver medallist Khachanov hit back, moving to two set points in the 12th game of the third set, cutting the match deficit when Ruud buried a lazy forehand in the net.

It was a brief setback, however, for Ruud who broke in the fourth set to lead 2-1, thanks to a pinpoint forehand pass, and backed it up with another break for 4-1.

- 'Humble enough' -

The Norwegian moved to three match points and sealed victory courtesy of a sweetly-timed drop shot with Khachanov rooted at the back of the court.

"After Roland Garros, I was extremely happy but at the same time humble enough to think that could be my only final in a Grand Slam in my career," said Ruud.

"They don't come easy. So here I am a couple of months later – it feels beyond words to describe."

Khachanov described the 55-shot rally which decided the opening set as "crazy".

"I have never played a point like that before but I felt pumped and in the game," said the 26-year-old.

"It was a painful way to lose a set. I felt OK after it but a few games later...maybe not!"

Alcaraz is the youngest Grand Slam men's semi-finalist since the 2005 French Open when Nadal won the first of his 22 majors.

The 19-year-old is also the youngest US Open men's semi-finalist since 1990 when Pete Sampras claimed the title.

However, the swashbuckling Spaniard needed two five-setters in the last two rounds to make his maiden semi-final at the Slams, spending more than nine hours on court to see off Marin Cilic and Jannik Sinner.

Against Italy's Sinner, he saved a match point in a quarter-final which stretched to five hours and 15 minutes and ended at a tournament record late finish of 2:50 a.m. on Thursday.

"I would say it was the best match I've played in my entire career," said Alcaraz who will become the youngest men's world number one of all time if was to go on and win Sunday's final.

Tiafoe, the world number 26, is bidding to become the first American Grand Slam men's finalist since Andy Roddick at Wimbledon in 2009.

The son of immigrants from Sierra Leone, Tiafoe is also just two wins away from becoming the first African-American champion in New York since Arthur Ashe in 1968.