11 and 16 that set up eagle putts of 55 and 10 feet.
Fast starts and tough finishes were the norm.
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.
"I was just a little exhausted, so I just kind of took the day off", he said after opening with a 70.
McIlroy, who was diagnosed with a fractured rib in January and didn't play again until March at the WGC-Mexico Championship, said the injury is not as painful this time; he is even-par through 36 holes at The Players Championship after birdieing his final hole for 71.
The three highest scores in the field on Thursday were produced by 2016 Masters champion Danny Willett (79), Scott Piercy (80) and Harris English (84), who had four double bogeys and a triple bogey on his scorecard.
Rickie Fowler and Adam Scott are also three shots behind McGirt and Hughes. "I don't know that you can really learn this golf course, because with this place if you hit one that lands four or five feet off the putting surface, there's no telling what's going to happen to it".
The two have a one-shot lead over Rahm, veterans J.B. Holmes and Chez Reavie and rising Swedish star Alex Noren.
Masters champion Sergio Garcia salvaged his opening round in the Players Championship with a hole-in-one at the iconic 17th at Sawgrass. Jordan Spieth opened with a 73, including a double-bogey on the first hole (his 10th) when he became so frustrated by a poorly raked bunker that he took a picture of it.
"It was a matter of having a good day", McGirt said of his round.
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"But swinging it like I'm swinging it, that's when it kind of fires the back up.
It's probably one of the hardest 125- or 140-yard shots that we get all year". McGirt had a pair of eagles on the back nine for his 67.
"Hopefully the guys in the lead don't finish more than they did yesterday, somewhere around nine under would be nice so we're three or four shots behind and we'll need a good round, but we'll have a chance".
It doesn't take much for this course to get under a player's skin. "I was, I think, in worse than a plugged lie, when it just trickled in".
"I didn't have any other shot", he said. "So it was a frustrating time in the round where I was trying to kind of get some momentum going".
Day was still just three shots back after he signed his first-round card. Harris English never got anything going and wound up with an 84.
At one under par McIlroy was eight shots off the pace and five behind 54-year-old Fijian Vijay Singh, who was looking to surpass the 52-year-old Sam Snead as the oldest victor in PGA Tour history with his first victory since 2008.
"I only played nine holes here".
"I was thinking actually 7-, 8-under after the second hole", said Day, who began the day on the 10th hole.
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