Kaymer holds a three-shot lead over former champion Northern Ireland's Graeme McDowell, Brendon de Jonge of Zimbabwe and Americans Kevin Na and Fran Quinn.

McDowell, who won the US Open at Pebble Beach in 2010, and Na had showed impressive form earlier in the day to top the leaderboard, carding rounds of 68 to finish on three-under par.

However, as conditions became more favourable in North Carolina, 2010 US PGA champion Kaymer took advantage and shot the lowest ever US Open round at Pinehurst's Number Two course, beating the 66 shot by Sweden's Peter Hedblom in 2005.

Negotiating the front nine holes in one-under, Kaymer picked up a further shot at the 10th before birdies on the 14th, 16th and 17th and an ice-cool six-foot par putt on the last saw him finish on five-under.

"It's only the first round and usually the golf course only gets more difficult," said 29-year-old Kaymer, who won last month's Players Championship at Sawgrass, Florida.

"So I think if you stay around level par, you can't be that far away from winning the golf tournament.

"It's nice to lead right now.

"It [the golf course] was more playable than I thought.

"I think that made a big difference mentally, that you feel like there are actually some birdies out there, not only bogeys."

Graeme McDowell is one of four players who trail Kaymer by three shots going into the second round at Pinehurst ©Getty Images



World number two Henrik Stenson of Sweden leads a group of players on one-under, four shots off the lead, that includes 2011 US PGA champion Keegan Bradley.

Justin Rose won the US Open at Merion Golf Club last year and the Englishman is looking to become the first player to defend the title since American Curtis Strange in 1989.

However, he did not get off to the best of starts and was four-over at the turn but picked up two shots on the back nine to card a 72.

"I have got to play some great golf for the rest of the week," admitted the 33-year-old.

"I have to do that pretty much no matter if I shot 66 today or 72.

"If I shot 66 I wouldn't be here thinking: 'Well, that's won me the tournament.'

"And 72 certainly hasn't lost me the tournament."

Rose was playing in a group with compatriot and US Amateur champion Matt Fitzpatrick and five-time major winner Phil Mickelson of the United States.

Justin Rose got off to a stuttering start as he attempts to defend his US Open title
©Getty Images



Fitzpatrick got off to a great start and shared the overall lead early on but dropped three shots in his last five holes to finish on one-over-par while Mickelson carded a level-par 70.

"That was great, I really enjoyed it," said Fitzpatrick, who will turn professional at the Irish Open in County Cork next week.

"They are both great guys and both really supportive if I hit a good shot."

World number one Adam Scott of Australia got round in 73, leaving him eight shots off the lead, while pre-tournament favourite and 2011 champion Rory McIlroy of Northern Ireland signed for a round of 71 that included three bogeys and two birdies.

The 2014 US Open at Pinehurst runs through until Sunday (June 15).

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