PINEHURST, N.C. -- Playing on a course that lacked an expected afternoon bite, Martin Kaymer impressed even himself with an opening-round 5-under 65 on Thursday at the U.S. Open on Pinehurst No. 2.

"Five under par is exceptional," said the German, who has a three-shot lead over Kevin Na, Brendon de Jonge, Graeme McDowell and Fran Quinn. "It's a great start to one of the most important weeks of the year."

It was also surprising. Playing in one of the final groups of the day, Kaymer figured the recently renovated No. 2 -- now without rough but still possessing its treacherous greens -- would only become more difficult as the day went on. Conditions changed from overcast and relatively cool in the morning to hot and sunny in the afternoon -- a classic recipe for increasingly difficult course conditions.

"It was quite nice to play late on Thursday, because you can watch (on television) in the morning and try to adjust mentally," Kaymer said. "I thought that it's going to be very firm in the afternoon. But actually it was more playable than I thought."

Kaymer, the winner of the 2010 PGA Championship and this year's Players Championship, said his confidence only grew as the afternoon wore on. He separated himself from Na, de Jonge, McDowell and Quinn with birdies on the 14th, 16th and 17th holes.

On the par-3 17th, he went straight at the flag, which sat in the back-left of the green with a bunker close by. He pulled out a 6-iron and decided to hit a high draw. It rolled 10 feet from the hole, and he made the birdie putt.

"That's the plan, to hit that high draw and I hit a perfect shot," Kaymer said. "It only adds confidence. That's the right way to play golf, I think. If you hit a bad shot, at least I tried it."

De Jonge, playing in the afternoon as well, was also surprised by the relatively benign conditions.

"It was noticeably softer out there," said de Jonge, who shared the lead before Kaymer's late surge. "But it can get pretty fiery out there. They can make it as difficult as they want it to be."

Ten players, including Brandt Snedeker, Matt Kuchar, Keegan Bradley, Jordan Spieth and Dustin Johnson, were tied for sixth, four shots behind Kaymer.

Snedeker had perhaps the day's most intriguing round. He shot 31 on the front nine -- marked by four birdies -- and was 4 under at one point. He then went bogey, double-bogey, bogey on Nos. 10-12 on the way to a 38 on the back nine.

"I didn't give myself any hard putts on the front nine," said Snedeker, who played in the morning. "Conversely, on the back nine, I had a lot of tough putts that I didn't handle well. The greens got faster and I didn't adjust."

  • Phil Mickelson, who would complete golf's "career slam" with a win at the U.S. Open, had an even-par 70. "The greens were soft," said Mickelson, who played in the morning. "The driver feels really good. The one club that's hurting me is the putter."

  • Defending champion Justin Rose was tied for 50th at 72. Masters champion Bubba Watson was tied for 122nd after a 76. "The golf course is better than me right now," Watson said. Adam Scott, the world No. 1, had a 73.

  • Stanford's Maverick McNealy closed out his 4-over 74 with a birdie on the 179-yard ninth hole, making about a 5- to 6-foot putt. Stanford's Cameron Wilson, the 2014 NCAA individual champion, shot a 78. Matthew Fitzpatrick and Oliver Goss were low amateurs at 71.

  • No evidence was found that Mickelson was involved in questionable stock trading in shares of Clorox, multiple sources told the New York Times. He remains under investigation over well-timed trades involving Dean Foods stock that netted him nearly $1 million, according to Times sources. "I'll continue to say, I haven't done anything wrong," Mickelson said. "I'm willing to help out, love to help out any way on the investigation."

    The Associated Press and wire services contributed to this report.

    LEADER BOARD
    After first round
    At Pinehurst No. 2; par 70
    Player Score
    Martin Kaymer 65 -- -5
    Kevin Na 68 -- -2
    Graeme McDowell 68 -- -2
    Brendon de Jonge 68 -- -2
    Fran Quinn 68 -- -2
    Notables
    Phil Mickelson 70 -- E
    Rory McIlroy 71 -- +1
    Full leader board, PAGE 7