South Africa's Trevor Immelman carded a second straight 68 to retain his overnight lead on a warm, breezy day two of the 72nd Masters at Augusta.
The 28-year-old fired five birdies and a bogey to climb to eight under, one clear of American Brandt Snedeker (68). Steve Flesch, Phil Mickelson and Ian Poulter ended five under with Paul Casey and Stephen Ames four under.
Tiger Woods shot 71 to edge back to one under, while co-overnight leader Justin Rose collapsed to 78 for two over.
Rose, 27, ran up a triple-bogey eight at the par-five 15th after finding water.
World number one Woods, who is chasing a fifth Masters title and a 14th major in all, said: "It was quite a fight to try to figure out what was going on with the wind.
"When it died on 14 it made things a lot easier. I've got to make a few more birdies, eliminate the mistakes and stay patient. Anything can happen and anyone can come back pretty quickly here."
Americans Stewart Cink (69) and Arron Oberholser (70) and champion 2003 champion Mike Weir (70) of Canada ended three under with two-time US Open champion Retief Goosen (71) and England's Lee Westwood (73) on two under.
"The course is going to show its teeth - you've got to make some good scores while you can"
Immelman, the 2006 PGA Rookie of the Year, returned to competition in January just six weeks after surgery to remove a non-cancerous tumour from his diaphragm.
"I realised that it can be taken away from you real fast," said Immelman, who also missed part of 2007 with a stomach parasite.
"I feel like I've been loaned a talent. I'm trying to do as well as I can.
"To shoot 68 the first two days is probably beyond my expectations. So I'm pretty thrilled right now."
The forecast for Saturday is heavy rain and possible storms and Immelman admitted he was relieved to post a competitive score.
"The course is going to show its teeth," he said. "You've got to make some good scores while you can."
Flesch shot a 67, the lowest round of the tournament so far, Poulter added a 69 to his opening 70, while two-time champion Mickelson picked up four shots in a 68.
Poulter, whose highest major finish is ninth at the 2006 USPGA, said: "Playing the way I am, I am feeling pretty confident.
"I've played really solid, holed the right putts at the right time and you have to have a little bit of luck, as I did at the 16th on Thursday."
Mickelson added: "I thought I played very well the first two days. I'm not disappointed. I gave myself a lot of easy pars, but I'm not leading so I've got some work to do."
England's Casey also shot a three-under 69, while countryman Lee Westwood remained in touch with a 73 for two under despite a double bogey at the par-five 13th.
Making it four Englishmen in the top 15 was debutant Nick Dougherty, who fired four birdies in his last six holes to add a 69 to his opening 74 for one under alongside Fiji's Vijay Singh and American Jim Furyk.
"I was rewarded for my patience. It's just having the discipline to maintain that state of mind in a major and I'm pleased I showed that," said Liverpudlian Dougherty.
The cut fell at three over, meaning the likes of Ernie Els, Sergio Garcia, Luke Donald, Fred Couples and Jose Maria Olazabal all missed out on the weekend.
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But Open champion Padraig Harrington made it on one over, while 50-year-old former champions Ian Woosnam and Sandy Lyle also squeezed in at two and three over respectively.
Woosnam, the 1991 champion, made his first cut in the Masters since 2000, a year after he feared he might never play in the event again because of back and health problems.
"It's nice being a senior - you just hit the ball and don't get nervous," said the former European Ryder Cup captain.
"I'm really, really happy. I keep getting spasms, but as I get going I get better."
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