Jimenez matched the lowest score ever shot by a player age 50 or older at the Masters, a 6-under 66 accomplished just twice previously by Ben Hogan in 1967 and Fred Couples in 2010, who is currently four strokes back and tied for 10th.
A victory for Jimenez would make him the oldest Masters winner, surpassing Jack Nicklaus (age 46 in 1986), and the oldest major champion, beating out Julius Boros (age 48 at the 1968 PGA Championship).
"That would mean a lot," said Jimenez, who has won 20 times on the European Tour. "I have plenty of victories in my career and having a major in my career would be amazing. That would be the flower on top."
Spieth was barely exempt for the PGA Tour at this time a year ago. A victory at the John Deere Classic earned him many things -- nearly $900,000, a spot in the following week's Open Championship and a two-year tour exemption. But biggest of all to him was an invitation to the Masters.
"This is the place I've always dreamt about," Spieth said. "I thought it was a good fit for my game. So far, so good."
He could become the youngest major winner since Tom Creavy captured the 1931 PGA Championship at age 20.
"He's young," Watson said. "Nerves are no big deal to him.
"I've won one, so I've got that going for me. If I play bad I still have a green jacket. We're all trying to win the same thing, we all know what it means for our careers and our status in the game."