American veteran Fred Couples and countryman Jason Dufner lead the Masters with Rory McIlroy and Lee Westwood one shot back after an absorbing second round at Augusta.
The 52-year-old Couples, the 1992 champion, shot 67 to join Dufner (70) on five under to head a group that also includes Sergio Garcia (68), former Open champion Louis Oosthuizen (72) and Bubba Watson (71).
First-round leader Westwood slipped to a one-over 73, while McIlroy inched closer with a 69.
Tiger Woods appeared in danger of unravelling as he sought to control his swing and ended with a 75 for three over, but Phil Mickelson gave chase with a 68 to climb to two under.
World number one Luke Donald flirted with the cut but carded 73 to head into the weekend at four over.
Sixty-three players made the cut, which came at five over, with only Westwood and McIlroy from the world's top 10 in the first 26 places on the leaderboard.
On a day that began overcast, chilly and breezy and ended in sunshine it was Couples' early 67 that stole the show.
The course specialist, who led with a first-round 66 two years ago, began his run with three straight birdies from the seventh before picking up further shots at 15 and 17.
"You know, I've said it for 28 years, this is my favourite golf tournament in the world," said Couples, who finished tied third in 2006 and tied second in 1998.
"The game plan was not try to do a whole lot of crazy things, just hit the ball solid, and I ended up shooting a lot lower than I thought. Five under was an incredible round."
The laid-back American is the oldest player to lead the Masters at halfway, beating Lee Trevino who was 49 when he jointly led with Nick Faldo in 1989.
Jack Nicklaus is the tournament's oldest ever winner at 46 in 1986, while 48-year-old Julius Boros is the oldest winner of any major.
Westwood reached six under after a birdie on the 15th and was leading going down 18 but a double bogey at the last dropped him back, while Dufner, the 2011 US PGA runner-up, was also six under before a bogey at the last.
"It was a disappointing way to finish," said Westwood. "If you get out of position on this course it can punish you, but these things will happen to everybody.
"I thought I made a few putts that did not drop but I'm right in there for the weekend and that's where I want to be."
US Open champion McIlroy put himself in an ideal position going into the weekend after getting up-and-down for a crucial par at the last.
"I drove the ball better. That was the big thing," he said.
"I think the whole round on Thursday was important to me; to not let the start get to me, and those two birdies at the end really gave me some momentum going into [today]. And to sort of stay patient and hang in there, I felt like I did that pretty well."
Woods was a shadow of the player who won the Arnold Palmer Invitational at Bay Hill a few weeks ago - let alone a 14-time major champion - as he wrestled to contol the new swing he is working on with coach Sean Foley.
The four-time champion pulled a succession of drives and cut a dejected figure as he struggled with his irons and missed a host of short putts.
The 36-year-old began brightly enough with two birdies in his first three holes but then bogeyed both the par threes on the front nine as well as the ninth for an outward 37. He failed to birdie any of the four par fives, and bogeys at the 11th and short 16th, where he almost hit back into the water from the bunker, cost him a 38 coming home.
"I didn't quite have it with my swing and unfortunately I had to hang in there and be patient," said Woods, who won his last Masters in 2005. "I didn't play the par threes very well or the par fives but if I just clean up those I'll be under par.
"One of the neat things about this tournament is the 10-shot rule [those within 10 shots of the lead make the cut]. Anyone can still win the tournament if they make the cut. I just need to cut the deficit down with a solid round and get off to a quick start on Sunday like I did last year."
Three-time champion Mickelson birdied the last for the second day running and is optimistic for the weekend.
"I played well and shot something in the 60s that got me back in the tournament," he said. "I was hoping for one or two more under, but that birdie on 18 felt terrific to finish that way. And to be only three back now heading into the weekend feels great, too. Especially after the first 10 holes yesterday."
Oosthuizen dropped from four under to one under and then climbed back up with birdies on 15, 16 and 17 to end where he began, while Garcia shot his best score for 12 rounds at the Masters as he looks to improve on a best finish of tied fourth in 2004.
"I'm just delighted to be in a pretty good position at the moment," said the 32-year-old, who last won in America in 2008 and has struggled for several years. "I wish I could tell you I'm ready to win, but I really don't know."
Scotland's Paul Lawrie began three under but was back to level par by the 15th before birdieing the last three holes to end in a group on three under.
Paul Casey (+7), Darren Clarke (+10), Ian Woosnam (+10), Simon Dyson (+11) and Sandy Lyle (+20) were among those to miss the cut.
Australian Jason Day, who was joint runner up with countryman Adam Scott last year, pulled out with a recurrence of an ankle injury.
