Stunning shotmaking on Saturday, including back-to-back eagles and nearly a third in a row from Mickelson, created classic roars from spectators at Augusta National Golf Club and raised expectations for a thrilling finish.
"That's what major championships are all about," said Westwood, who seeks his first such crown. "They are tough ones to win because great players do great things."
Westwood, trying to become the first Englishman to win a major since Nick Faldo rallied from six strokes down on the final day to beat Greg Norman in the 1996 Masters, led the field at 12-under par 204, one stroke ahead of Mickelson.
World number one Woods, in his first event after a five-month hiatus since his secret sex affairs were revealed to the world, shared third on 208 with South Korean K.J. Choi, one stroke ahead of 50-year-old US star Fred Couples.
"The guys on the leaderboard are great players. They are going to do something," Westwood said. "You have to expect the unexpected at times."
Westwood, whose near-miss majors include third at last year's British Open and PGA Championship and the 2008 US Open, might need something unprecedented.
No one has fired four rounds in the 60s at any Masters, something Westwood is poised to accomplish. He plan is simple - keep his focus on his game as he has all week
"What Phil is doing is of no importance to my game," Westwood said. "What Phil Mickelson does is really out of my control. The only thing I can control is what I do, where I hit it."
Tricky pin placements will test the world's best, especially at Amen Corner, the perilous par-4 11th, par-3 12th and par-5 13th holes winding around Rae's Creek, where dreams of glory vanish as misfired balls splash into the water.
"I think it's going to be an exciting Sunday," Mickelson said. "I think the course is going to be set up similarly in that you can make some birdies and challenge some pins."
Westwood, who leads the field with 43 of 54 greens in regulation, will duel with Mickelson, a three-time major winner and two-time Masters champion, in the final last-day pairing, which has produced 18 of the past 19 Masters champion.
"We're going to have some excitement, a real shootout," Mickelson said.
More history is against Woods in his quest for a 15th major title and fifth Masters green jacket. Woods has never won a major when he was not leading after 54 holes.
"I'm only four back. I'm in good shape," Woods said. "You never know."
A first-ever comeback major victory would move him closer to the record 18 major titles won by Jack Nicklaus heading into the US Open at Pebble Beach and British Open at St. Andrews, majors at places where Woods has won prior majors.
A victory rally would complete a storybook comeback for Woods, who began the week uncertain how he would be received by spectators at Augusta National after more than a dozen women claimed sexual affairs with the married father of two.
Woods received applause and a generally warm welcome, although the numbered badges of Masters patrons allow for future credential removal for hecklers, meaning Woods might not be out of the woods when it comes to PGA Tour events.
"After what has transpired, to have that support out there is fantastic," Woods said.
Couples is trying to turn back the clock and become the oldest champion in major golf history while Choi is quietly contending for Asia's second major win in a row, after countryman Yang Yong-Eun in last year's PGA Championship, while playing alongside Woods all four days.