Phil Mickelson moved a step closer to one of his biggest goals, a U.S. Open title, as he emerged from a see-saw third round battle at Merion to take a one-shot lead into Sunday's final round.
Mickelson rolled in a nine-foot birdie putt at the 254-yard 17th hole 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 but still came away with the lead after completing 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."
Sunday could be a dream day for Mickelson, who would add a missing line to his impressive resume with an Open victory as he celebrates his 43rd birthday and Father's Day with his dad watching him play at Merion.
"I've had opportunities in years past, and it has been so fun, even though it's been heart breaking to come so close a number of times and let it slide," Mickelson said.
One shot back after a rousing battle atop 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 top spot 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 in charge.
The big left-hander said he felt better placed to tackle the challenge of winning the U.S. Open this time around.
"I feel better equipped than I have ever felt heading into the final round of a U.S. Open," said Mickelson, who has three Masters and a PGA Championship title to his credit.
"My ball striking is better than it's ever been. My putting is better than it has been in years, and I feel very comfortable on this golf course. I love it.
"Given that I've had some past major championships now and some success here...I think it's going to be fun. I don't think I feel any more pressure than anybody else who wants to win."
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 also moved within three shots of the pace with birdies on the first hole.
However, the high-profile duo, playing their third successive round together, staggered through the rest of the way. Woods shot 76 to plunge 10 shots off the pace, and McIlroy registered 75 to be nine behind Mickelson.
Padraig Harrington shot a round of 75 to leave him alongside Woods on nine over, while amateur Kevin Phelan's 78 leaves him on 16 over.
After the merry-go-round of leaders in the front nine, Schwartzel and Donald settled into a tie for supremacy after the turn and were joined at two-under by Mahan when he birdied the 16th.
Mahan and Schwartzel both bogeyed the last two holes to yield the top position to Donald, who was then overtaken by Mickelson.
Schwartzel said he knows what it will take for him to add a second major title Sunday to his Augusta triumph.
"You're going to have to give every single shot all your attention," the South African said. "There's no hole where you can sort of ease back and sort of freewheel a bit. Every shot that you hit is really intense."
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 the best round of the day.
Senden, who briefly led by one, faded with five bogeys in the last 11 holes to post a 74, finishing six strokes off the lead.
Coming up big at the end was Mickelson, who was clearly excited about what Sunday could mean.
"It's got the makings to be something special, but I still have to go out and perform and play some of my best golf," he said.