* Watson gunning for second crown in three years
* Spieth in position to be youngest Masters champion
* Jimenez could become oldest to don green jacket
Spieth, playing with patience and discipline belying his years, posted a solid two-under-par 70 to join Watson on five-under 211 on a warm, sunny day at Augusta National that baked the famed greens to lightning speeds.
"Patience," Spieth said about the key to his round. "We could tell early on the greens were ridiculous. You had to put the ball on the right spots and not let your focus stray for one moment."
Spieth, who began the day four behind overnight leader Watson, put himself in position to make Masters history on Sunday as victory would make him Augusta's youngest champion, putting him ahead of Tiger Woods who was 21 when he triumphed in 1997.
Watson, the 2012 champion, needed to curl in a four-foot par-saving putt at the last to retain a share of the lead on a day when he struggled on the heavily contoured greens.
"It was the firmest I've seen it in years out here," said Watson who only missed one fairway but needed 33 putts.
"I had a couple of three-putts. If you two-putt those you're right there and you've got a two-shot lead.
"All in all, a good day," Watson said, looking on the bright side. "If somebody told me on Monday I'd have a 74 and still be tied for the lead I'd have taken it all day long."
One stroke back were Matt Kuchar, who used some brilliant chipping to register a 68, and Masters first-timer Jonas Blixt of Sweden (71).
Another shot adrift on 213 were American Rickie Fowler (67) and 50-year-old Spaniard Miguel Angel Jimenez who had seven birdies and one bogey in a tournament-low 66.
An eagle at the par-five second hole helped Watson improve his 36-hole lead to a five-shot cushion but that disappeared under a spell of bogeys that turned the year's first major into a taut battle on a crowded leaderboard.
Watson wobbled with three bogeys in four holes from the fourth, striking delicate chip shots and sand blasts too far while leaving some downhill putts short in fear of rolling well by.
The poor stretch dropped Watson into a tie with Blixt and Thomas Bjorn of Denmark at five under with nine holes to play before the American moved back in front with a birdie at the 10th.
Ten players were bunched within two shots of the lead at the turn. Blixt also reached five-under after back-to-back birdies from the 15th but a bogey at 17 dropped him back and Spieth emerged as co-leader.
At the other end of the age spectrum, Jimenez could put his name in the Masters record books as the oldest champion, surpassing Jack Nicklaus who was 46 when he won in 1986.
Jimenez vaulted into contention by matching the lowest Masters score by a player over 50 held by Ben Hogan (1967) and Fred Couples (2010).
"I feel very nice. Beautiful day out there, just light breeze sometimes. It's great," said the pony-tailed Spaniard.
Three shots behind the co-leaders were England's Lee Westwood (70), Bjorn (73) and American Jim Furyk (72).
Americans Couples, 54, and Kevin Stadler, U.S. Open champion Justin Rose of England and Australian John Senden were tied at one under par heading into what could be a wild shootout for the year's first major.
Another record-tying performance was turned in by long-hitting American Gary Woodland who took advantage of his early start on greens not yet baked to a crisp to fire a six-under 30 on the front nine and tie the Masters record.
Woodland had four birdies and an eagle to match the record low for the outward half by Johnny Miller (1975), Greg Norman (1988), KJ Choi (2004) and Phil Mickelson (2009).
He climbed within three shots of Watson before stumbling at Amen Corner as he ended with a 69 for 216, even par.
Adam Scott, last year's winner, struggled on the greens and posted a 76 for 217.
The next test for young Spieth will be contending with the pressure cooker that is Masters Sunday.
"He's young, nerves are no big deal to him," said Watson of Spieth who seems destined for greatness after winning the U.S. Junior Amateur in 2009 and 2011, joining Tiger Woods as the event's only multiple winners, and who became the youngest PGA Tour winner in 82 years last season at the age of 19.
Still, the final round at Augusta is something special.
"You know the roars, you know the history of the game," said Watson. "As a kid we've always dreamed about Sunday afternoon, having a chance on the last hole or birdying the last four like Charl Schwartzel did in 2011.
"On Sunday that's a big deal because that's the day they give out the green jacket so it makes all of us nervous and makes all of us sweat a little more and makes every putt mean that much more."