The world number six bounced back from a bogey on the first with birdies on the fourth, sixth and ninth, while a bogey on the 10th was followed by a hat-trick of birdies from the 11th and an eagle on the par-five 16th.
Day, who has been dealing with injuries and personal issues relating to his mother's cancer treatment, jumped out to a fast start.
Defending champion Jason Day ran off two straight birdies after making the turn and was in the lead at 5-under, which for the former world No. 1 was a peculiar position.
Past year s Masters champion Danny Willett shot a 79.
"I wouldn't have expected a round like that", said Hughes, who has missed four of his last six cuts.
Rickie Fowler and Adam Scott are also three shots behind McGirt and Hughes.
"If I can manage to shoot 4- or 5-under [Friday], I'm still going to have a great chance", he said.
Alex Noren (68), Jon Rahm (68) and Lee Westwood (70) did not make bogey either.
First, Scott watched Masters champion Sergio Garcia salvage his poor round by making a hole-in-one at the infamous island green.
This is the fifth time McGirt has held the lead or a share of the lead after 18 holes, but he has yet to convert an opening round lead into a victory.
"I went back at it again on Friday [May 5] and instead of maybe gradually building up again, I hit balls for four or five hours and did the same on Saturday", he said.
"It was a matter of having a good day", McGirt said of his round. "There's no answer, honestly; you just kind of see the shot and hit the shot and see the putt and hit the putt". The Players Stadium Course must be easy, huh.
Teeing off from the back nine, Day was bogey-free with three birdies but the Australia star faltered late. The course definitely played tougher later in the day. "I was, I think, in worse than a plugged lie, when it just trickled in".
"I'm excited about where the state of the game is", Day said.
"It feels more like a muscle strain so I'm going to get an MRI scan on Monday after the tournament and I might have to rest for a week or two".
England's Ian Poulter was a shot further back as he looked to capitalise on retaining his PGA Tour card after fellow professional Brian Gay alerted officials to a discrepancy in the points structure used for players competing on major medical extensions. Harris English never got anything going and wound up with an 84.
The last player to win the Masters and record a hole-in-one in a PGA Tour event in the same year was Tiger Woods in 1997.
"I wouldn't have expected a round like that, but I knew my game was trending", said the 26-year-old from Ontario, like McGirt, a one-time PGA Tour victor. The eagles were at No. 11 and No. 16, and it was the first time in tournament history that one player accomplished that feat in the first round.
Thursday was one of those exceptions.