The 21-year-old American stood on 14-under 130 after two rounds, a bogey-free Friday giving Spieth a Masters midpoint record-matching five-stroke lead over compatriot Charley Hoffman.
"Any time you can set a record here is pretty awesome," Spieth said. "I'm really excited about how I played."
Fourth-ranked Spieth broke the Masters 36-hole low of 13-under 131 set by Ray Floyd in 1976 and nearly broke the low two-round major start of 130.
"Didn't know what any of these scores meant in history or anything like that," Spieth said. "I just knew I had a good look at birdie and had a good read on it, and it was just barely off."
Spieth, seeking his first major title after a runner-up Masters debut last year, shared the biggest 36-hole lead mark with Floyd in 1976, Jack Nicklaus in 1975 and Herman Keiser in 1946, after Hoffman closed a round of 68 with his lone bogey.
Spieth pitched from 50 feet to inches from the cup to set up a birdie at the par-five second, sank an 18-foot birdie putt at the fifth and birdied eight after finding a fairway bunker. Spieth made an 18-foot birdie putt at 10 and sank eight-footers for birdie at the par-5 13th and 15th.
Spieth's 36-hole domination, with 15 birdies against one bogey, evoked memories of Tiger Woods' record-setting 1997 Masters romp for his first major title. Woods will remain the youngest Masters champion by about five months even if Spieth wins this year.
