Day, who started the day just three off the pace, has dropped a distant 11 shots back thanks to playing his opening 16 holes two-over par, leaving him three-under and tied seventh.
American Charley Hoffman is the only player currently putting any sort of heat on Spieth, sitting at 10-under through 16 holes.
Ernie Els (72) and Paul Casey (six holes) are third, already nine shots back.
Day opened his round with a three-putt bogey on the first but he seemingly wiped out the problem by hitting a stunning approach to five feet on the par-five second before coaxing in the eagle putt.
He held firm for three holes with pars, the last an impressive save from a tough bunker lie, but then three-putted the sixth green for another bogey just as Spieth pushed forward with birdies.
Leaving his approach on the seventh short, despite a great drive, compounded his misery and the 27-year-old failed to get up and down from the sand to drop another shot.
There were bogeys at 11 and 14 before a birdie on the 15th stopped the rot.
Spieth notched birdies on the second, fifth, eighth, 10th, 13th and 15th holes and had a six-foot look on the 18th that, had it gone in, would have given him the major championship records all on his own.
Meanwhile, Tiger Woods produced his best golf of the year, firing a three-under-par 69 in the second round of the Masters to finish at two under.
- AAP
