The 19-year-old defeated Casper Ruud 6-4 2-6 7-6 (7-1) 6-4 on Sunday to ensure he pipped his opponent to top spot in the rankings, becoming the youngest man ever to do so.

Alcaraz is more than a year younger than Lleyton Hewitt was when he topped the standings in 2001 and set the previous record.

The Australian's pre-eminence in the sport proved short-lived as first Roger Federer and then Rafael Nadal ascended to the top of the game, but in the week where King Charles assumed sovereignty, tennis appears to have crowned a new king.

"Right now I'm enjoying the moment," Alcaraz said.

"I'm enjoying having the trophy in my hands.

"But of course I'm hungry for more. I want to be at the top for many, many weeks. I hope many years. I'm going to work hard again after these amazing two weeks. I'm going to fight to have more of this."

It is only a year since Alcaraz burst into the spotlight by reaching the quarter-finals at Flushing Meadows but his talent was already well known within the sport and he has climbed the rankings at warp speed.

He showed that he was ready to win the biggest titles by lifting Masters trophies in Miami and Madrid, where he beat Nadal and Novak Djokovic back to back, and has proved himself to be a complete player despite his tender years.

Alcaraz has also shown remarkable powers of physical endurance, recovering from three successive five-set matches.

He spent 23 hours and 40 minutes on court during the fortnight, another record, and at times he appeared physically and mentally weary during the final but Ruud, who was heavily beaten by Nadal in his maiden slam final at the French Open, could not find the killer blow.

The Norwegian appeared to have turned the tide in the third set and brought up two set points at 6-5 but Alcaraz bravely came to the net on both occasions to save them.

Ruud then played an awful tiebreak and Alcaraz found his second wind to claim victory in the fourth.

"It's crazy for me," Alcaraz said.

"I've never thought that I was going to achieve something like that at 19 years old. So everything came so fast. It's something I dream (of) since I was a kid, since I started playing tennis."

Alcaraz arrived in New York with a few doubts after early losses in the ATP events in Montreal and Cincinnati and he thanked his coach, former world No.1 Juan Carlos Ferrero, and his team for helping to change his mindset.

"I felt the pressure. I couldn't smile on court," he said.

"I came here just to enjoy ... I love playing tennis, of course. I would say if I smile, if I have fun out there, I play my best level, my best tennis."

Alcaraz was in tears on his chair after the match, and he revealed he was thinking about his mother and his grandfather, who did not travel to the US.

The teenager uses the mantra 'brain, heart, courage', and he said: "These things mean a lot to me because my grandfather told me (them) since I was a kid in every match, in every tournament. This is something that I remember in all the tough moments."

Australian Associated Press