Spain’s next great tennis star has officially arrived, with Carlos Alcaraz winning the US Open and becoming the youngest world No.1 in ATP history.

The 19-year-old condemned fifth seed Casper Ruud to his second straight defeat in a grand slam final with a 6-4 2-6 7-6 (7-1) 6-3 victory in New York on Sunday local time.

Alcaraz spent nearly 14 hours on court in his previous three matches but defied the horrendous physical toll to outlast Ruud across nearly three and a half hours in the final.

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The teenager collapsed to the court and broke down in tears before a warm embrace with his 23-year-old opponent was followed by a climb to reach his team.

At 19 years and four months old he is now the youngest world No.1 in history, surpassing Australia’s Lleyton Hewitt (20 years and eight months).

Alcaraz’s grand slam victory follows his idol Rafael Nadal – who wasted no time celebrating the feat – winning this year’s Australian and French Open.

“Congratulations Carlos Alcaraz for your first grand slam and for No.1, which is the culmination of your first great season, which I am sure will be many more!” Nadal tweeted.

“Great effort Casper Ruud! Very proud of you! Tough luck today but amazing tournament and season! Keep going!”

Carlos Alcaraz celebrates during his maiden grand slam final victory. Credit: Getty

Ruud has now lost his first two grand slam finals, having been thrashed by Nadal at Roland Garros in June.

Inside Arthur Ashe Stadium on Sunday, Alcaraz dropped the second set and faced a pair of set points while down 6-5 in the third.

But he erased each of those one-point-from-the-set opportunites for Ruud with the sorts of quick-reflect, soft-hand volleys he repeatedly displayed.

And with help from a series of shanked shots by a tight-looking Ruud in the ensuing tiebreaker, Alcaraz surged to the end of that set.

One break in the fourth was all it took for Alcaraz to seal the victory in the only grand slam final between two players seeking both a first major championship and the top spot in the ATP’s computerised rankings.

– with AAP