By the time Hunter Mahan reached the 18th green at Congressional Country Club — normally a pleasant place to be as the shadows lengthen — the crowds had long since thinned. Elsewhere in Bethesda and beyond, there were air conditioners, swimming pools, showers, all means of comfort. But the fairways on the Blue Course Friday in the the AT&T National were cruel, unpleasant places to be.

“Golf is a challenge in itself,” Mahan said, “and when the conditions and the weather comes into play, it’s a whole ’nother factor. Once your mind goes, the body is going to go with it. It’s very important to be mentally strong.”

So those there to greet Mahan by the completion of his superb second-round 65 were only the toughest — or masochistic — fans. But they saw him curl in a 12-footer for his final birdie to push his lead to 7 under par. It is an especially low number given conditions that gave players headaches and had them seeing stars. The Blue Course was at its fiery best Friday, hard and fast, and only Brendon de Jonge, Jimmy Walker and Robert Garrigus enter the weekend within two of Mahan.

So the question: If the weather stays the same — and it’s supposed to be similarly stifling — could the leading number on Sunday night be the same as it was on Friday?

“If it stays like this,” said Tiger Woods, five back after a 68, “it’s not going to go anywhere.”

If it stays like this, who will be standing at Congressional? At 4 p.m. Friday, the temperature reached 100 degrees, the heat index an oppressive 109. If any Washington golf fan under the age of 50 ever wondered what Ken Venturi endured on the legendary sweltering day he won the 1964 U.S. Open at Congressional, he could have found out Friday.

“I don’t know how much worse it can get out there,” said Rod Pampling, who shot 67 and sits three back. “It’s ridiculous.”

Put the heat together with a stout course set-up, and it could seem ridiculous. Consider that last year, when Congressional staged the U.S. Open — annually billed as the toughest test in golf — those who shot 5 over for two rounds missed the cut. This year, in a regular PGA Tour event, those who shot 6-over 148 made it. That 148 total was matched at this year’s U.S. Open, but it is the highest cut line on a regular tour event since the Barclays in August 2009.

What’s more? It could get harder.

“If there’s no rain,” Garrigus said, “it’s going to be something else.”

All that makes Mahan’s round — matched in this tournament only by Cameron Tringale, who is four back — more impressive. The 10th-ranked player in the world has pleasant memories here, what with his final-round 62 in 2009, the last time this event was staged at Congressional, a score that was only good enough to lose to Woods by one. The vibe, though, was positive.

“I really enjoy playing here,” he said.

It showed. Mahan is an exceptional ball-striker, the kind of player who can tackle “punishing” (as he called Congressional) layouts with precision and focus. He made an excellent up-and-down par-save at 14 by chipping the ball beyond the hole and letting it run back. And he said he was “proud” of his par save at 17, because he got up and down after his worst iron shot of the day.

“It wasn’t a hard round, I guess,” Mahan said, a sentiment expressed by few Friday. “I hit so many fairways and greens, I made it easy on myself.”

Woods might not say the same. The highlight from his day was undoubtedly his 50-foot, double-breaking, left-then-right eagle putt he rolled in at the par-5 16th, which he played as his seventh hole. That took him from 1 over to 1 under, and he was in it, a strong pump of the right fist to prove so. But the crux of his round may have actually come on the two previous holes.

“The pars at 14 and 15 were something I needed to have happen,” Woods said.

These were not, though, play-to-the-center-of-the-green, two-putt pars. At 14, Woods hit his tee shot into the right rough, where it settled down deep, as if trying to hide from the searing sun. He had to come down on it at such a steep angle, and with such force — “I’m hitting it as hard as I can,” he said — that his right leg kicked out after impact, a little dance move.

The ball shot out, a classic low, U.S. Open-style, save-yourself-from-disaster liner. It settled in the fairway, 76 yards from the pin, and Woods still had work. He handled that beautifully, striking a wedge to three feet and saving par.

The 15th wasn’t much different. This time, the drive was left, into the gallery. This time, he punched the approach to 90 yards from the pin. This time, he hit his wedge to 15 feet. And this time, he made a longer, more difficult putt. But the result was the same: a par when bogey seemed imminent, and it kept his round alive.

“I’m right there,” Woods said.

That is what is left after a testy, torrid day at Congressional: Mahan and Walker, de Jonge and Garrigus are among those ahead of him. But Woods is right there, trailing one of America’s best young players, exactly the element that can keep this tournament from wilting in the heat.