Juan Carlos Ferrero outfought Andre Agassi in a thrilling match at Flushing Meadows to reach the US Open final and seal the world number one spot.
Ferrero raced through the opening two sets and quelled a brave fightback by Agassi to seal a 6-4 6-3 3-6 6-4 win in an electric atmosphere in New York.
Ferrero, never past the fourth round in four prior attempts, called the victory his greatest ever on a hardcourt surface and said he was delighted to have taken over from Agassi as world number one.
"I didn't play so good for a couple years in the US Open - now I'm playing very good," he said.
"I worked a lot for this number one. It's my first time and I'm going to work really hard to stay there."
Ferrero produced some scintillating tennis in the first two sets and Agassi, the top seed, looked to be heading for a comprehensive straight sets defeat.
But the New York crowd roared in appreciation as the 33-year-old performed a remarkable turnaround in the third set.
Ferrero had chances to break at 3-3 but Agassi dug deep and against the run of play he forced a break and took the match into a fourth set.
As the match appeared to be swinging in the top seed's favour, Ferrero looked increasingly frustrated but as Agassi failed to make the most of his momentum, the 23-year-old struck.
The American netted a forehand to give Ferrero the opportunity to serve for the match and the French Open champion held his nerve to clinch a momentous win.
"I thought I did everything pretty well except serve in the first two-and-a-half sets," said Agassi.
"I wasn't getting any free points on my serve. He was just taking care of his business better than I was.
"He had the freedom to take more chances with the groundstrokes. By the time I was getting into the match I was already down two sets.
"Just a little too big of a hill to climb."