Hunter Mahan (Getty Images)
Hunter Mahan surged to the top of the AT&T National leaderboard with a six under par 65 which put him two strokes clear of the field at the halfway point in Maryland as Noh Seung-yul moved into contention.
Mahan's round of seven birdies and a bogey on another stifling day at the Congressional course put him seven under overall, two clear of two fellow Americans, Jimmy Walker and Robert Garrigus, as well as Zimbabwe's Brendon de Jonge.
Behind them on four under was a group of five which included Noh, Stewart Cink and Vijay Singh, while Tiger Woods carded a much-improved 68 to sit ominously on two under alongside overnight leader Bo Van Pelt.
With temperatures reaching 109 degrees Fahrenheit, Mahan said mastering the conditions was key.
"When the conditions and the weather comes into play, it's a whole other factor," he said.
"I've got to stay mentally tough. Once your mind goes, the body is going to go with it. It's very important to be mentally strong.
"We've got two more days of this, so it's going to be important to take care of yourself every night and every day when you're out on the golf course, or it's going to cost you shots."
Woods sunk a 48 foot eagle putt on the 16th hole, helping him move into a tie for 11th.
Woods, starting five shots behind overnight leader Van Pelt, went three under par to reach a two under total of 140.
Woods told www.pgatour.com: "It's just one of those days where you just stay patient.
"You shoot something in the 60s, and I think that would have been a good score. I thought it was a very good score. I'm right there."
Van Pelt hit a disappointing second round of 73, including six bogeys, while Scotland's Martin Laird made a move with a fine two under 69 to sit one under overall.
Defending champion Nick Watney was one behind Laird after going one over.
England's Greg Owen was four over for his round but made the cut on three over, the same mark as Beau Hossler, the 17 year old amateur who made headlines at the US Open.