FLUSHING MEADOWS, N.Y.: From the moment he saw a picture of Arthur Ashe Stadium, the world’s biggest tennis venue, Juan Martin del Potro dreamed about winning the US Open.

But he was just a child. And he was from Argentina ­— land of clay-court champions, not hard-court masters.

Monday night, the 20-year-old del Potro collapsed flat on his back on that very court — all 6-feet-6 of him — and broke down in tears, having upset the man regarded as the greatest to play the game, Roger Federer, to win the title he coveted most, 3-6, 7-6 (7-5), 4-6, 7-6 (6-4), 6-2.

It took del Potro 4 hours 6 minutes to claw back from a two-sets-to-one deficit and claim his first major.

In doing so, he denied Federer a sixth consecutive US Open and record-extending 16th major titles.

But he hardly sullied Federer’s otherwise spectacular year — one in which he reached the final of all four majors; achieved a career Grand Slam by winning his first French Open; claimed the Wimbledon title; married his long-time girlfriend and became a father of twins.

“Five (US Opens) was great,” said Federer, 28. “Four was great, too. Six would have been a dream, too. Can’t have them all. (The) year has been amazing already, and it’s not over yet. Got married and had kids. Don’t know how much more I could want.”

While Federer will remain No. 1 when the world rankings are next computed, del Potro will supplant American Andy Roddick at No. 5.

But the Argentine proved he’s destined to rise further still after toppling the two men who have dominated the sport for the last five years, Rafael Nadal and Federer, in a 36-hour span en route to Monday’s title. Del Potro was as gracious in victory as he was resilient through the fight.

“I have two dreams in this sport: One is (to win) the US Open; and the other is to be like Roger,” del Potro said during the on-court trophy presentation.

“One is done,” he added, turning to Federer. “But I need to improve a lot to be like you. I would like to congratulate you. You are a great champion.”

Federer had no trouble complimenting his young rival afterward.

“He hung in there and gave himself chances,” Federer said. “In the end, he was the better man.”

Federer wasn’t at his best Monday. His serve lacked its customary accuracy. His groundstrokes erred more often than usual. Even his impeccable comportment was missing in the tense third set, when Federer complained about the length of time del Potro was taking in deciding to challenge line calls and then cussed chair umpire Jake Garner after he tried to quiet him.

“Don’t tell me to be quiet, okay?” Federer snapped. “When I want to talk, I talk!”

Then, in response to something Garner said, Federer barked, “I don’t give a (expletive) what he says.” Federer wasn’t the only man on edge.

Del Potro said he was so nervous about his first major final that he had trouble sleeping Sunday night and couldn’t eat breakfast Monday.

His anxiety showed in the first set. His footwork was off, and he struggled to place his first serves. Federer broke early and claimed the set in 40 minutes. But once del Potro steadied himself, he gave Federer his stiffest challenge of the tournament, blasting forehands that sounded like cannon blasts.

Once a liability, del Potro’s serve has become a weapon. And Monday he produced his best when he needed it most, fending off 17 of Federer’s 22 break points.

The rain-delayed final, played for a second consecutive year on a Monday afternoon, was hardly flawlessly played, but tightly contested to the delight of a near capacity crowd.

The second set was settled by a tiebreak, in which del Proto capitalized on a forehand error by the Swiss and then crushed a backhand winner down the line to create three set points. After squandering two, he closed it to draw even at one set each.

Del Potro broke for a 4-3 lead in the third set on successive forehand errors by Federer.

But the Swiss broke back and held serve to take a 5-4 lead. That’s when Federer lost his temper, annoyed that the chair umpire was allowing del Potro to ponder whether to challenge calls for so long.

“I take 2 seconds; he takes like 10,” Federer barked from his courtside chair. “Do you have any rules, or what?”

The verbal sparring seemed to sharpen Federer’s focus while costing del Potro his. The Argentine coughed up successive double-faults to get broken and hand Federer a two-sets-to-one lead.

That was the low point of the match for the Argentine.

Then he reminded himself that he was in the final of a Grand Slam event, facing the best player in history — a man he’d never beaten in six previous attempts.

“Nothing to lose,” del Potro told himself.

So he started playing with more abandon, investing even more power in his forehand.