Carlos Alcaraz defeated Frances Tiafoe in a gladiatorial U.S. Open semi-final on Friday, setting up a showdown for the title and world number one ranking against Casper Ruud.

The 19-year-old Spaniard triumphed 6-7 (6/8), 6-3, 6-1, 6-7 (5/7), 6-3 to become the youngest men's Grand Slam finalist since compatriot Rafael Nadal captured the first of his 22 Slams at the 2005 French Open.

Norway's seventh-ranked Ruud earlier defeated Russia's Karen Khachanov 7-6 (7/5), 6-2, 5-7, 6-2 to also reach his first Grand Slam final.

Tiafoe went down fighting, however, saving three match points and retrieving breaks in both of the last two sets.

"We are in the semi-final of a Grand Slam, we have to give everything we have inside, we have to fight until the last ball," said Alcaraz, the youngest U.S. Open finalist since Pete Sampras in 1990.

"It doesn't matter if you're fighting for five hours or six hours. It doesn't matter, you have to give everything on court."

For Alcaraz, who unleashed 59 winners, it was his third successive five-setter as he closes in on a maiden Slam and becoming the youngest ever world number one.

"It's my first time in a final of a Grand Slam. I can see the number one in the world, but at the same time it's so far away," he added.

"I'm going to give everything that I have. I will have to handle the nerves of being in the final of a Grand Slam but obviously I'm really happy."

Tiafoe hailed his conqueror. "I gave everything I had, too good from Carlos tonight," he said. "Honestly I came here wanting to win the U.S. Open, I feel like I let you guys down. This one really hurts."

Ruud will be appearing in his second Grand Slam final of the season after finishing runner-up to Nadal at the French Open in June.

"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."

The 23-year-old Norwegian set the tone for his dominance early in the semi-final when he came out on top in a 55-shot rally to convert a third set point in the opening tiebreak which his Russian rival described as "crazy".