Day is hopeful but not certain to be fit for the third round.
Read more: Day collapses at US Open
Spieth reeled off four birdies in his opening eight holes to jump to the outright lead at six-under but handed it back with a double bogey as he made the turn.
From there birdies on the first and ninth holes were only tempered by a seventh hole three-putt bogey.
"I know that it's going to get tougher and tougher now that [the weekend] hits," Spieth said as he looks to be just the sixth player to win the Masters and US Open in the same year behind Tiger Woods (2002), Jack Nicklaus (1972), Arnold Palmer (1960), Ben Hogan (1951, 1953) and Craig Wood (1941).
"I'll draw some on Augusta, but at the same time my patience level has to be even that much higher. I'm not quite putting myself in the same positions off the tee, so I've got to be a little more methodical.
"At Augusta I was kind of finding fairways, hitting it on the green and I was making everything. That would be nice here if I could do that, but it's a harder golf course than the Masters played this year."
Playing with Spieth, Day had two bogeys and just one birdie on the backside of the course before an incredible birdie on the first, his 10th.
After a greenside bunker shot went across the green and down a massive slope Day was able to hole out for a miraculous birdie.
"I screamed. I had a good view," Spieth said of Day's hole out.
"It was a cool shot to see. I probably wouldn't scream if he did it on Sunday and we were tied, but it was nice today."
Day backed it up with a 30-foot birdie putt on the second and was just two off the lead when his bogey on four was tempered by a birdie on eight.
But the collapse and subsequent bogey soured an otherwise good round.
-AAP