Day made a pair of long birdie putts and otherwise put on a clinic at Bay Hill for a 7-under 65 that gave him a five-shot lead among the early starters and set a daunting target for everyone else going into the weekend.
''It was great,'' he said. ''I felt like I couldn't do anything wrong out there.''
Day was at 13-under 131 and was five shots ahead of Jamie Lovemark, who had another 68. Paul Casey had a 69 and was six shots behind.
Henrik Stenson and Justin Rose were among those playing in the afternoon who tried to catch Day, or at least keep it close. But the PGA champion was driving it long and straight, hitting his irons well and making big putts. It's a great combination for anyone, particularly a player who reached No. 1 in the world last year by winning four out of six tournaments, including his first major.
Rory McIlroy was able to watch the whole show.
McIlroy, who opened with a 75, was in the group behind Day and saw the Australian start to pull away from the field. McIlroy had his own issues on Friday, starting with a chance to play for two more days. He handled that with ease, making four birdies on the back nine to get above the cut line and posting a 67.
But he was all but ready to concede that the tournament was out of his reach.
''I was looking at Jason in front of me and I was thinking if I could maybe get within six going into the weekend,'' McIlroy said. ''And now it's 11. ... Even playing well, I'm not sure that's quite going to be enough that far behind Jason. At the same time I can get confidence from that and bring it on to the next week and ultimately into the Masters.''
Stanford junior Maverick McNealy, chosen to play by his colleagues at the Palmer Cup, had a 71 and was at 4-under 140. It's the second straight week that an amateur has made the cut; Georgia senior Lee McCoy finished fourth last week in the Valspar Championship.
U.S. Amateur champion Bryson DeChambeau was playing in the afternoon.
Day, however, appeared to be in his own world on the immaculate Bay Hill course.
A pair of two-putt birdies on the par 5s on the back nine helped him extend his lead, and he added a 35-foot birdie putt on the 17th. It was after Day had to scramble for par on the par-5 fourth hole, making a 10-foot putt, that he took off. He rolled in a 10-foot birdie on the next hole, got up-and-down from a bunker for birdie on the par-5 sixth and finished his round with a 35-foot birdie on the ninth hole.
''The total that he's on now, 13 under, I thought that was going to have a great chance at the end of the week,'' McIlroy said. ''I thought something around 12 under was going to be right there. For him to do that after two days is pretty spectacular. I think everyone has got their work cut out to try and catch him.''
The timing is good for Day, who had a sluggish start to the year after taking three months off at the end of 2015. He has only Bay Hill and the Dell Match Play next week before the Masters, and he thought it would be helpful to at least feel the pressure of contention before Augusta National.
The way this was shaping up, the question was whether anyone could put serious pressure on him.
Big leads are never easy. Adam Scott found that out two years ago when he lost a seven-shot lead on the week. Day, however, has recent experience. He opened with a 61 in the BMW Championship, stretched his lead to five shots and then six shots going into the final round and won by seven.
''At the BMW, I showed a lot of patience,'' he said. ''I shot 18 under the first two days and shot 4 under the next two days, which is very boring golf. But I won, which was great. I'm not saying that I'm going to do what I did the last two days at BMW. I've got to think my way around this golf course. I've got to make sure that I'm trying to be smarter than everyone else. My game plan has to be on point.
''I have to make sure that I'm out there not to try and get soaked up in much of his and go to single focus,'' he said. ''The ultimate goal is to win the tournament.''