Phil Mickelson and fellow American Billy Horschel retained their one-stroke lead at the U.S. Open following Saturday morning's conclusion of the suspended second round of the championship at Merion.

Four-times major winner Mickelson and 26-year-old Horschel, who won the Zurich Classic in April for his first PGA win, finished their second rounds on Friday and were tied at one-under-par 139 heading to the third round on a warm, sunny day in the leafy Philadelphia suburb.

One stroke back at level-par 140 were former world number one Luke Donald, who shot a second-round 72, Justin Rose (69) and American Steve Stricker (69), who all registered their rounds on Friday.

Donald was joining Mickelson and Horschel in the final third-round grouping at 2:40 p.m. (1840 GMT) as the 73 players to make the cut, set at eight over par, were going off in threesomes from the first and 11th holes.

Lurking well within range on three-over 143 were Tiger Woods and Rory McIlroy, who will playing their third successive round together, joined by Gonzalo Fernandez-Castano of Spain.

Padraig Harrington sits in a share of 20th place, on four over par, while Irish amateur Kevin Phelan made the cut, finishing in a large group on eight over who qualified for weekend play.

Sixty-eight players returned at 7:15 a.m. (1115 GMT) to complete their rounds, suspended by failing light as the tournament continued to be backed up after losing more than four hours on Thursday's opening day due to lightning and downpours.

Many contending players lost ground in Saturday's resumption as lively breezes blew across the classic layout, although 2011 Masters champion Charl Schwartzel of South Africa and five-time PGA Tour winner Hunter Mahan improved their position.

Schwartzel emulated Mickelson and Horschel with a birdie at the difficult par-four 18th, playing at 501 yards, to complete a 71 and move up to one-over-par 141 for the tournament.

Mahan, who had started his second round from the 11th, birdied his last hole, the 10th, and also improved by a stroke in posting 69 to join Schwartzel on 141. The pair will play together in the third round, along with Belgian Nicolas Colsaerts (72).

Englishman Ian Poulter, who had stood level par with four holes to play, bogeyed the par-four 16th Quarry Hole and the daunting par-four last for a 71 to slip to two over par.

"I think 18 is probably the hardest hole in golf right now," said Poulter. "I played a lovely three-wood and three-iron and it ran just off the back edge of the green and I didn't get up and down."

Amateurs Pan Cheng-tsung of Taiwan and South Korea-born Michael Kim, both surprisingly in the top 10 when second-round play was suspended, also moved in the wrong direction on Saturday.

The 21-year-old Pan plunged from even par to four-over 144 in completing his last nine holes for 72 on Saturday, while the 19-year-old Kim lost two strokes to par over his final seven holes to dip to three-over-par 143.