NEW YORK - Casper Ruud reached the US Open final and closed in on the world No. 1 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 Spanish teenager Carlos Alcaraz in Sunday's championship match.

The 19-year-old ended Frances Tiafoe’s dream run at Flushing Meadows with a 6-7 (6-8), 6-3, 6-1, 6-7 (5-7), 6-3 win to reach his first major final.

It will be world No. 7 Ruud's second Grand Slam final of the season after finishing runner-up to Rafael Nadal at the French Open in June.

Despite playing into the early morning hours in his previous two five-set matches, the 19-year-old showed no signs of fatigue. He fell on his back and covered his face with his hands after winning the hard-fought battle.

If he can beat Ruud, Alcaraz would become the youngest man to be world No. 1, breaking the mark set by Australian Lleyton Hewitt, who was 20 when he became the world’s top-ranked player in 2001.

The win ends Big Foe’s run at the tournament, where his unlikely success and uniquely American life story captivated fans and brought out luminaries including former first lady Michelle Obama to Arthur Ashe Stadium on Friday night.

The stakes of Sunday’s final could not be higher. The winner will walk away with his first Grand Slam trophy and the world No. 1 ranking.

Ruud has an enjoyed a more straightforward and less-talked-about route to the final but will be the underdog against Alcaraz, who is 2-0 against the Norwegian including a win at the Miami Open final in April.

“He’s playing really, really well,” Alcaraz said.

“I know that. I will have to show my best.”

Had Alcaraz lost his semi-final, the 23-year-old from Norway would have become the new world No. 1 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."

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

“I feel great right now,” a beaming Alcaraz told reporters after the win, which set up a final with Norway’s Casper Ruud on Sunday.

“I mean, a little bit tired. But right now I’m just so, so happy.”

Alcaraz’s run at the tournament last year came to a jarring finish when he retired from his quarter-final with a leg injury he said came from playing long matches in his previous rounds.

That turn led some to wonder if the highly-touted teenager possessed the endurance needed to win a Grand Slam given its grueling, best-of-five format. But he has more than silenced the doubters with three consecutive five-set victories on the sport’s biggest stage.

“I’ve played more matches in five sets, I am more prepared mentally and physically,” Alcaraz said.

“It was 12 months of working hard in the gym, on the court.

But I would say it’s all mental."

If he wins on Sunday, he will be the second-youngest US Open champion after American Pete Sampras, who claimed the first of his five titles in 1990.

However, the swashbuckling Spaniard needed two five-setters in the previous 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 2.50am on Thursday.

Tiafoe, the world No. 26, was 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 was also just two wins away from becoming the first African-American champion in New York since Arthur Ashe in 1968. AFP, REUTERS