Spieth carded his worst round of the week so far, a two under par 70, and a lead of seven at one point on the back-nine was cut to four by the time he putted out on the 18th.
His worst mistake came at the 17th, where a wayward drive and a poor chip led to a double-bogey six, and he was lucky to escape further damage at the last.
A poor approach shot landed in the middle of the crowd, but Spieth produced a fine chip from the other side of a bunker and holed from inside eight feet to keep his advantage over a charging Justin Rose.
“I thought two under would be a good score today but obviously being four??under and closing at two under is disappointing, but it could have been worse??and I was very pleased with that up and down on 18,” said Spieth, whose 54-hole total of 200 provided him with another Masters record.
“It took some guts to hit that flop shot and the par putt was huge, one of the??biggest putts I have ever hit.
“I was very frustrated. I didn’t do any scoreboard watching until the 15th and??then knew I was six clear and pars were good scores on the last two holes.
“Driver should never have come out of my bag at that point. I was getting a??little erratic with the driver and could have hit three-wood, eight iron.
“It felt a little bit different out there. It was weird.
“I played in the morning yesterday and I had more than 24 hours from the time I finished to the time I teed off and that’s a long time to sit on a lead like that.??Maybe I got a little bit anxious at times.”