Juan Martin Del Potro of Argentina returns to Rafael Nadal of Spain during the Mens Singles Semifinal of the 2009 U.S. Open. (Jared Wickerham/Getty Images)
Federer, 28, has beaten del Potro in all six career meetings but the 6-foot-6 (1.98 metres) Argentine has been one of the hottest players in the world during the hardcourt season.
The two met at this year's French Open semi-final and Federer escaped with a five-set win en route to the title. The Swiss could see del Potro was finding his game.
"Every month he was getting better," said Federer. "He was hitting his serves better, from the baseline he was getting more confident.
"He knows much more now today what he needs to do on the tennis court."
The 20-year-old Del Potro ruined for Open fans an anticipated Federer match-up with Rafael Nadal, by blitzing the Spaniard 6-2 6-2 6-2 in their semi-final on Sunday.
"I'm very happy to beat Rafa in straight sets, played an unbelievable match," said Del Potro, who admitted the Open is his favourite tournament. "Of course it's great for me and for my future being in finals.
"I'm very close to do it. I've always dreamed of this moment."
The Argentine is a dangerous hardcourt player, one who hits the ball flat but extremely hard with sharp angles.
"Del Potro had a great match today against Rafa, because Rafa doesn't give you any easy matches," said Federer, shooting for his 16th grand slam win. "So for him to come through the way he did, it's, to say the least, impressive.
"That's why he's not to be underestimated."
Federer is the heavy favourite but Novak Djokovic, who lost to the Swiss 7-6 7-5 7-5 on Sunday, thinks it would be foolish to overlook the sixth-ranked Argentine.
"Del Potro is showing his best tennis in last couple of months, and you've got to give him credit for that," he said. "He's still a young guy, but moving extremely moving well for his height and is an all around player.
"He has a good chance against Roger if he plays his best. Why not?"
