World number one Federer, bidding to become the first man to win three Grand Slams in the same year since Swede Mats Wilander in 1988, dismissed British fifth seed Tim Henman 6-3, 6-4, 6-4.
Earlier, 2001 champion Hewitt extinguished the fire of Sweden's Joachim Johansson with a clinical display of percentage tennis to win 6-4, 7-5, 6-3.
Swede Johansson, the boyfriend of Hewitt's sister Jaslyn, took his ace tally for the tournament to 123 as he boomed 17 thunderbolts past Hewitt, but the Australian ruthlessly pounced on his opportunities when they came along.
The first came as 28th seed Johansson's forehand temporarily deserted him at 4-5, allowing Hewitt to take control.
Fourth seed Hewitt was rarely threatened in the second set, biding his time before breaking Johansson in the 12th game, prompting a trademark fist pump.
The former world number one managed a single break in the third set to wrap up a routine victory and reach his first grand slam final since winning Wimbledon in 2002.
"It's awkward playing somebody you know, and who you hit with regularly, but you just have to put your professional hat on when you get out there," said Hewitt, who trained with the Swede at the family home in Adelaide last Christmas.
"(Jaslyn) was going to be happy for someone and disappointed for someone no matter what. But in years to come we will probably have this situation a lot," Hewitt told reporters.
"Joachim's already made a few comments to me in the locker room, he said he's not buying me a Christmas present this year, he said that was it today."
NO FAVOURS
Johansson, who ended the hopes of defending champion Andy Roddick in the quarter-finals, said he had not expected any favours from Hewitt, who committed only 14 unforced errors in the one hour 59 minute contest.
"He is always pumped and there was no reason for him not to be today," said Johansson, who was playing in his first grand slam semi-final.
"I didn't play as well as I was hoping, I missed a few easy shots and got broken three times from the same end," added Johansson, the first Swede to reach the semi-finals at Flushing Meadows since Jonas Bjorkman in 1997.
Federer was troubled initially by Henman, a player the Swiss trailed 6-2 in head-to-heads. But after clinching the first set he moved through the gears to race away from the 30-year-old Briton, who was aiming to reach his first Grand Slam final.
"Obviously I'm really pleased to be in my first U.S. Open final," Australian Open and Wimbledon champion Federer said.
"It's going to be tough against Lleyton, we always have good matches, but I think I've won my last 10 (finals) so I hope I can keep it up."
Federer can become the first man in history to win his first four Grand Slam finals if he beats Hewitt on Sunday, and Henman said he would not be betting against the Swiss.
"He's playing phenomenal tennis right now," said Henman, who was playing in his sixth Grand Slam semi-final.
"I think he's got every chance to go down as one of the best ever. If you take (Andy) Roddick's serve, (Andre) Agassi's returns, my volleys and Hewitt's speed and tenacity, then you've probably got a good chance against Federer."
Hewitt will not have that luxury on Sunday but, with a 7-5 lead in their previous battles, the gritty Australian believes he can end the top seed's charge.
"I lost to Roger at the Aussie Open and Wimbledon (this year) but I played extremely well. I'm going to have to play some great tennis to get on top of him.
"But I feel like I'm playing well and I give myself a good chance. It's going to be a huge match."
