Spieth moved to the top with the aid of birdies at the 14th and 15th holes and finished three rounds with a 5-under-par score of 211. Watson, seeking his second Masters title in the last three years, struggled with his putting and with some yardages but converted pars on the final two holes for a 74 and gained a spot with Spieth in Sunday's final pairing.
Behind the co-leaders, 13 players are within 5 strokes of first place. Matt Kuchar fired a 68 and stood at 212 with Jonas Blixt, who held a share of the lead after nine holes Saturday. Fifty-year-old Miguel Angel Jimenez carded the best round of the week, a 66, to tie Rickie Fowler at 213.
Former Lancaster resident Jim Furyk (72-214) and reigning U.S. Open (at Merion) champion Justin Rose (69-215) are among those ready to pounce on Sunday. But initially, the attention will be on Spieth, who had no status on the PGA Tour 15 months ago and now is the world's 13th-ranked player.
In addition to his bid to become the first Masters rookie to win the tournament since Fuzzy Zoeller in 1979, he can break the record of Tiger Woods, who was 21 years, 3 months, and 14 days old when he won the 1997 Masters. On Sunday, Spieth will be 20 years, 8 months, and 17 days old.
Spieth did a fine job of holding himself together on greens that he called "crazy, crazy fast. . . . I've never putted on greens like this before." He hit the ball solidly, stayed patient, and made par after par thanks to his excellent lag putting.
And, oh yes, he talked to himself.
"I'm 20 and this is the Masters, and this is a tournament I've always dreamt about," he said. "Like Mr. [Ben] Crenshaw has always said, it brings out more emotion than ever in somebody.
"As far as being patient shot-to-shot, I think I've done the best that I ever have with my mental game. But yeah, I'm still going to talk to myself out there. Really a lot of it is just kind of guiding myself, trying to pump myself up and be really positive, which is abnormal. I mean, typically I'm quiet, or if I'm talking, I'm asking [my caddie], 'What's going on?' "
Spieth was at even-par for the day when he reached the 14th hole, where his approach shot to a back hole location stopped three feet away and he made birdie. He also drained a six-footer for his final birdie of the day at No. 15, and he converted lengthy 2-putts for pars at the 16th and 18th holes.
"I wanted to get in with pars on the last three to get into the final group," he said. "I'm very pleased to have that opportunity because no matter what, I can control my own destiny."
So can Watson, who held a 5-stroke lead after an eagle 3 at the second hole but carded just one birdie after that. For the first time, he failed to birdie both of the par-5s on the back nine, but pars on the last two holes put him in a positive frame of mind at the end.
"My wayward shots, or shots I wish I could do over, weren't left or right, they were just always long or short," he said. "But I'm in a good situation. There's a lot of people that wish they were in my situation, shooting 74."