Charl Schwartzel of South Africa grabbed a one-shot lead midway through Saturday's third round of the U.S. Open as contenders on the jam-packed leaderboard jockeyed for position at Merion.
Schwartzel, the 2011 Masters champion, was the last of six different players to grab a share of the lead during a see-saw front nine that shifted with every hole after the first, and became sole leader with a birdie at 10 to reach two under par.
The South African, who was three-under for the day with four birdies following a bogey at the par-five second, led former world number one Luke Donald of England by one stroke on a warm, breezy day in the Philadelphia suburb.
Six players were tied another two shots back at one over par, including 36-hole co-leader Phil Mickelson, his fellow-American Hunter Mahan, Australian John Senden, Englishmen Ian Poulter and Justin Rose, and impressive 19-year-old South Korea-born amateur Michael Kim.
Mickelson, halfway mark co-leader Billy Horschel, Senden, Donald and Rose were also in the lead at various times during the tumultuous outward half, producing a constant change in leadership in the day touring pros refer to as Moving Day.
Tiger Woods made up a stroke on the leaders with a birdie at the first before co-leaders Mickelson and Horschel teed off to move within three shots of the pace.
But the world number one, aiming to end a five-year drought in the majors, added five bogeys on his card as he struggled to keep the ball in the fairway and slipped to seven over par.
Woods's playing partner Rory McIlroy also birdied the first, but the Northern Irishman hit his tee shot at number two out of bounds and matched the slide by Woods to also stand seven over.
Suffering a notable implosion was Spain's Sergio Garcia, who hit three tee shots out of bounds on the 15th hole on his way to a six-over 10 at the par-four. Garcia was five over for the day and 11 over par for the tournament through 16 holes.