Hunter Mahan birdied five holes on the front nine to shoot a six-under 65 and take a two-shot lead after two rounds of the PGA Tour's $6.5 million AT&T National tournament.
With temperatures hovering around 103 degrees Fahrenheit (40 Celsius) at the Congressional course on Friday, Mahan had the hot stick.
The American finished 36 holes at seven-under-par 135 after shooting an opening round 70.
The 2009 runner-up won twice earlier this year, giving him a total of five career USPGA Tour victories. He is also fresh off a closing-round 61 at the Travelers Championship.
He birdied two of his final three holes to record the best round of the day.
Mahan kept it going on the back nine with six-straight pars before a birdie on the par-five 16th followed by another on the par-four 18th.
"When the conditions and the weather come into play, it's a whole other factor," Mahan 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."
Brendon de Jonge and Jimmy Walker are tied for second place at five-under after both shot two-under 69s. They were joined by Robert Garrigus, who posted a four-under 67.
Former major winner Stewart Cink shot a three-under 68 to move to four-under-par 138. He shares fifth place with Fiji's Vijay Singh (70), Australia's Rod Pampling (67), American Pat Perez (69) and South Korea's Noh Seung-yul (68).
First-round leader Bo Van Pelt dropped into a tie for 11th at two-under-par 140 after a two-over 73.
Tiger Woods moved up the leaderboard with a three-under 68 to join Van Pelt and two others at minus-two.
Woods, who had an eagle, bogey and two birdies, said he didn't mind the heat.
"I live in Florida where it is not quite this hot but it is definitely more humid every day. I played in other places like Malaysia where it is definitely more hot," said Woods, whose mother is from Thailand.
Woods faced similar sweltering conditions when he blitzed the field at the Southern Hills Country Club in Tulsa to win the 2007 PGA Championship.
"It's one of the reasons why I had success at Southern Hills, because I felt physically fit, didn't have a problem with it (heat)," Woods said.
"I've played some of my good tournaments over the years in Malaysia and other places where it's hot, and certainly fitness, running all those miles and lifting all those weights, it comes into play when you get days like this, and consecutive days like this.
"It's just one of those days where you just stay patient. I shot 68 today, which I thought was a very good score."
Woods won this event three years ago, finishing ahead of runner-up Mahan.
Defending champ Nick Watney of the United States managed a one-over 72 to fall into a share of 19th.