Biddle, a sophomore at the University of Nevada-Las Vegas, fired a bogey-free 67 at the par-72, 7095 metre Erin Hills course while Main, a three-time All-American at UCLA, shot a 5-under-par 65 at the par-70, 6055 metre Blue Mound Golf and Country Club in Wauwatosa.
Hahn, winner of the 2009 Western Amateur, carded his 5-under-par round at the Blue Mound Golf Club.
No one has successfully defended his US Amateur title since Tiger Woods won the championship for the third consecutive year in 1996. Uihlein beat Stanford's David Chung on his 21st birthday to win the 2010 US Amateur at Chambers Bay and enters the 2011 championship as the world's third-ranked amateur.
Whilst not getting too ahead of himself, Uihlein admits that he's lot wiser as to the events that lay ahead at these championships.
"I'll probably be a little less nervous when I tee it up than I am now," says Uihlein, who's father Peter is the CEO of Acushnet, the parent company of Titleist.
"I'm more comfortable with a club in my hand than talking about things I've done. "I don't like talking about things I've done. I just want to keep moving forward and get better."
Main, who was the runner-up at the 2010 Western Amateur, was the 2009 Pac-10 Conference Freshman of the Year and enjoyed himself during the opening round of this major championship.
"It was fun," said Main.
"I hit it really well. It was probably the best round I've had all summer. I started out one over through three holes and after that, made a bunch of birdies. I birdied four and then eagled five, so that was good, the eagle. Then I birdied nine, so I just got it going."
The four Australians in the field sees Daniel Nisbet currently in a tie for 21st position after a 2-under-par, 70 around Erin Hills, Matt Stieger in a tie for 38th position after scoring a 1-under-par, 69 at Blue Mound, Ryan McCarthy opened with a 2-over-par, 72, at Blue Mound, while Bryden Macpherson is at 4-over-par an in a tie for 199 after a 76 at Erin Hills.
The US Amateur continues Tuesday with a second round of stroke play, after which the field of 312 will be cut to 64 for head-to-head match play competition through Sunday.
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