Newly crowned US Open champion Juan Martin Del Potro admits he is not yet consistent enough to challenge for top spot in the world rankings.
The Argentine, who turns 21 next week, has risen to fifth following his epic 3-6 7-6 (7-5) 4-6 7-6 (7-4) 6-2 triumph over world number one Roger Federer in the final at Flushing Meadows.
Despite Del Potro ending his five-year run of US Open success, Federer kept hold of top spot, while Rafael Nadal's run to the semi-final in New York moved him back above Andy Murray into second.
A fourth-round loss against Marin Cilic means Murray is down to third.
"I have very good players from me to the number one," said Del Potro.
"For sure, Roger, Rafa, Murray, they are two or three steps more than me. I'm working hard every day hard to be like them.
"But if I play for many years in this level, then they are going to retire and I'm going to be much better."
Confident
Federer acknowledged the recent improvement he has seen in the 6ft 6in Del Potro's game.
"Every month he was getting better," Federer said.
"He was hitting his serves better, from the baseline he was getting more confident. He knows much more now what he needs to do on the tennis court.
"Whereas maybe in the beginning, because he's so tall, he needs to figure out his game, I think he did that in the last six months."
Del Potro is the only player other than Nadal to defeat Federer in a Grand Slam final, despite losing all six previous meetings with the Swiss master.
With Nadal recovering from injury problems and Murray turning out a disappointing US Open performance, Del Potro looks well placed to ascend the rankings further.
"I have everything to learn," he added. "I want to be a good player in the future. I want to be like them. I'm very happy I'm on my way.
"I just go my way and try to improve every day in my game. Of course I'm living much better than two years ago, maybe 12 months ago. I'm in good shape. I'm physically feeling good with my tennis. That's important."