* Mickelson aims for first U.S. Open title
* Three-way tie for second at Merion
Phil Mickelson birdied the long par-three 17th hole to seize the lead and will carry a slender one-shot advantage into Sunday's final round of the U.S. Open at the challenging Merion Golf Club.
Mickelson rolled in a nine-foot putt at the 254-yard 17th to leapfrog playing partner Luke Donald, who slipped out of the lead with a bogey on the mammoth par-three.
Four-times major winner Mickelson, looking for his first U.S. Open title after finishing runner-up a record five times, bogeyed the daunting 18th hole to complete an even-par 70 for a one-under-par 209 total.
"It is a hard challenge, but it is a lot of fun," said Mickelson, keen for the next step in his quest for a first U.S. Open crown.
"Let's go. I can't wait to get back out playing."
One shot back on a see-saw day at the top of the leaderboard were 2011 Masters champion Charl Schwartzel, who posted 69, and Americans Hunter Mahan (69) and Steve Stricker (70).
Former world number one Donald, who double-bogeyed the 18th after struggling with the deep rough to finish with a 71, was another shot adrift at one-over 211 along with fellow Englishman Justin Rose (71) and 36-hole co-leader Billy Horschel (72).
Mickelson, the leader after each round, fell from the lead after a bogey on the third hole, but came alive on the back nine with three birdies to fight his way back on a tumultuous day that saw a slew of competitors take turns at the top.
Six different players - Mickelson, Horschel, Donald, Rose, Australian John Senden and Schwartzel had turns in the lead over the first nine holes.
World number one Tiger Woods, hoping to end a five-year drought in the majors dating back to the 2008 U.S. Open, and world number two Rory McIlroy both moved within three shots of the pace with birdies on the first hole.
However, the high-profile duo, playing their third successive round together, both staggered through the rest of the way. Woods shot 76 to plunge 10 shots off the pace, and McIlroy registered 75 and was nine behind Mickelson.
Three shots off the pace was Australian Jason Day, who registered a 68 for two-over 212 despite a bogey at the last. One more shot away was Rickie Fowler who carded 67 for 213.
Senden, who briefly led by one, faded with five bogeys in the lasgt 11 holes to card a 74, finishing six strokes off the lead.