Pinehurst, United States – Martin Kaymer opened the second round of the US Open with birdies on two of his first four holes, stretching his lead to four strokes at rain-slowed Pinehurst.

Cool, overcast conditions and overnight rain helped the 2010 PGA Championship winner and other morning starters find softened landing spots on the layout's notorious turtle-backed greens.

Kaymer, coming off a victory last month at the Players Championship that ended a three-year win drought, birdied three of the last five holes Thursday to shoot a five-under par 65, the lowest US Open round ever at Pinehurst, and led by three as the day began.

The 29-year-old from Dusseldorf opened with a six-foot birdie putt at the 10th and birdied 13 as well, becoming only the third player in 37 years to lead by so much at a US Open.

Kaymer, a former world number one who slid from the top 60 in world rankings, found a bunker off the tee at 14 but sank a 12-foot par putt to keep his four-stroke edge.

Sharing second on three-under were Zimbabwe's Brendon de Jonge and 49-year-old US journeyman Fran Quinn with afternoon starters Graeme McDowell of Northern Ireland and American Kevin Na sharing fourth on two-under after 68s Thursday.

Quinn, whose handful of wins over 26 years as a professional include the 1999 Thailand Open, opened with a birdie to reach three-under after two holes.

De Jonge opened with a bogey but recovered with birdies at the third and fourth to stay within four of Kaymer.

Among a pack on one-under were Japan's Hideki Matsuyama and Swedish world number two Henrik Stenson.

Reigning British Open champion Phil Mickelson, a six-time US Open runner-up seeking a victory to complete a career grand slam, was on level-par 70 awaiting an afternoon start.

“I didn't give any away on the short ones but I didn't make any of those I needed to make from 15 to 20 feet to have a really good round,” Mickelson said. “I've got to make some of those that make the difference around here.”

Others still solidly in the hunt after opening on 70 include England's Paul Casey and Ian Poulter, Welshman Jamie Donaldson, France's Victor Dubuisson, South African Charl Schwartzel, South Korean Noh Seung-Yul and Australian Aaron Baddeley.

Northern Ireland's Rory McIlroy, coming of a breakup with tennis-star fiancee Caroline Wozniacki and a victory at Wentworth last month, opened on 71.

Defending champion Justin Rose of England was on 72 while world number one Adam Scott and fellow Aussie Jason Day each opened on 73.

Bubba Watson, who won his second Masters in three seasons in April, will struggle to make the cut after firing a 76 on Thursday.– Sapa-AFP