THIRD ROUND LEADERBOARD (US unless stated):
-12 L Westwood (GB) -11 P Mickelson -8 T Woods, KJ Choi (Kor) -7 F Couples -6 I Poulter (GB), R Barnes, H Mahan -5 A Kim, YE Yang (Kor)
Masters leaderboard (external website)
US Masters highlights - day three
By Rob Hodgetts
BBC Sport at Augusta
Lee Westwood leads Phil Mickelson by one shot going into the final day after an enthralling third round of the Masters at Augusta.
The Englishman fired a 69 to finish 12 under and stay clear of the charging American who thrilled the cheering galleries with back-to-back eagles.
Tiger Woods and KJ Choi are tied third at eight under with 50-year-old Fred Couples one shot further back.
Ian Poulter, who shared the second-round lead with countryman Westwood, carded a two-over 74 to slip back to six under.
Americans Ricky Barnes and Hunter Mahan are also at six under with compatriot Anthony Kim and US PGA champion YE Yang of South Korea one stroke adrift to round out the top 10.
Westwood, chasing a first major title, picked up shots at the first, fourth, eighth and 10th to reach 12 under and hold a four-shot lead over Poulter.
But Mickelson, the 2004 and 2006 champion, sparked a sensational 30-minute spell in which he hit eagles at the par-five 13th and par-four 14th - only the third time this has been done in Masters history. He followed up with a birdie on the 15th to get 12 under.
When Westwood dropped a shot at the short 12th, Mickelson was the outright leader before handing a shot back on 17 to finish 11 under.
Watch Mickelson's back-to-back eagles
"I haven't played this well in a long time and I feel like my game's as good as it's been," said Mickelson. "I love this tournament more than any other. I love Sunday being in contention. And I'm excited."
Westwood grabbed a shot back with a birdie at 15 and held on to his one-stroke advantage going into the final round.
"I'm delighted," he said. "I played really well out there. All aspects of my game were solid. I'm amazed how calm and collected I was out there. I know it's a position I wanted to be in, so I'm looking forward to it."
His playing partner Poulter admitted he will need to shoot a low number on Sunday to have any hopes of winning the title.
"I'm a few shots adrift of where I wanted to be and I'm not overly happy right now," he said. "But I have a chance. This course can give up eagles quite easily and I have to go out and try to do that."
Woods, playing in the penultimate group with Choi, had an up and down front nine but hit four birdies in the last six holes, marred by a bogey on the 17th.
"I was fighting it all day," he said. "I really struggled with the pace of the greens and I was fighting my swing.
"It was a tough day. I just wanted to put myself back in contention and I did that. At one point I was seven back and to fight back to only being four behind is a pretty big accomplishment. But we have a long way to go."
Westwood is bidding to become the first English Masters winner since 1996
Couples contributed to the excitement generated by Mickelson as he birdied the 14th and eagled the 15th before a bogey at the last dropped him to eight under.
"I love this place," said the 1992 winner. "I have a shot at it if I can shoot a crazy score. We'll see what happens."
Two-time winner Tom Watson, 60, carded a one-over 73 to end two under alongside 2008 winner Trevor Immelman.
"I was disappointed with the way I played today," said Watson, who won the Masters in 1977 and 1981.
"With no wind and perfect conditions a 73 is not a very good score. I didn't strike the ball very well.
"But it's still always a pleasure to walk around Augusta when the azaleas are popping and there was the most beautiful bluebird on 17 I ever saw."
Defending champion Angel Cabrera, who made the cut with no room to spare, carded a three-under 69 to finish level par, while 16-year-old Italian amateur Matteo Manassero ended four over in a group with world number two Steve Stricker and Sergio Garcia of Spain.