Carlos Alcaraz has beaten Casper Ruud in four sets to win his first Grand Slam title at the US Open in New York.

A captivating winner-takes-all encounter enthralled the fans at Arthur Ashe Stadium in Flushing Meadows, with the victor also guaranteed to climb up to number one in the world.

It was the 19-year-old Alcaraz who came out on top 6-4 2-6 7-6[1] 6-3 to end his remarkable breakthrough season with a memorable triumph.

The Spaniard, who becomes the youngest-ever men's world number one, took the opening set in 50 minutes, but soon saw the momentum shift in Ruud's favour.

The Norwegian appeared to suss out Alcaraz's drop-shot tactic and duly punished the teenager, who for a while looked short of other ideas as questions were raised over fatigue following a run to the final which saw him taken to the full five sets in each of the last three rounds.

Ruud was dominant on his way to a 6-2 win in the second to level things up, winning the final four games to close out the set.

Alcaraz responded with an early break to move into a 2-0 lead in the third, but Ruud once again hit back and a mammoth set went all the way to a tie-break, with both players thrilling the crowd through some incredible exchanges and rallies.

Alcaraz saved two set points in the 12th game to force that tie-break, and Ruud's form deserted him at the worst possible time as Alcaraz won seven points on the bounce.

A break in the sixth game of the fourth set proved decisive for the Spaniard as he moved to within touching distance of glory, and he took the match at the second available opportunity to become the first teenager to ascend to the top of the men's rankings.

Only Pete Sampras in 1990 has won the US Open at a younger age, while Alcaraz also joins an elite group with Arthur Ashe and Rod Laver as the only players to have won the tournament in one of their first two appearances in the main draw.

Ruud, meanwhile, is forced to settle for both second place on the night and in the rankings, rising up to world number two despite narrowly missing out on a first Grand Slam crown of his own.