Danny Lee was leading the B-Flight with a bogey-free round, but made only two birdies on the par-5s on the back nine.
World No.1 Dustin Johnson birdied three of his closing six holes in a 69 to be next best at nine-under but nine shots adrift of the lead.
But his tee shot at the par-three 17th hit the wooden bulkhead surrounding the green, soaring high into the air before hitting the surface and rolling into the drink. After his second shot ran over the back of the green and into a bunker, Simpson played a delicate shot that trickled just over a ridge in the middle of the green and clanked off the center of the flag and in for an eagle 3.
Tiger Woods will be sticking around for the weekend's action but only just scraped by the one-under cut-line, after rounds of 72 and 71, along with pre-tournament favourites Jordan Spieth and Justin Thomas.
Simpson was 11 under for the day when he stepped on to the 17th tee but his pushed tee shot on Sawgrass' signature hole clattered against a railway sleeper and ended up in the water. They still were 11 shots behind. - At this rate, it appears that no one can beat Webb Simpson, not even Pete Dye.
It sent Simpson to a remarkable 19-under par and then seven shots clear of his rivals.
With the cut flirting with two under, McIlroy was having to go hunting for birdies and, while that paid off with gained strokes at 12, 14 and 16, it cost him with a bogey in between at the 13th. The good news for Simpson?
"If you were to ask me, could somebody get to 19-under, I would have said that would be really tough, and the conditions have to be just right, " Simpson said. Half the time when I miss fairways here I end up thinking to myself "how have you done that, it's bloody massive!"More news: UFC 224 Start time Preview at Brazil
"I was with her last week in Charlotte, so this would be an fantastic Mother's Day present for her", he said.
"It's a lot of fun to play good golf, and hopefully I can keep that up".
Harold Varner III, who is tied to ninth after a two-under 70, added: "I think we all should be surprised". "But I'm trying not to dwell there".
Webb Simpson took a commanding five-shot lead heading into Saturday at the 2018 Players Championship, as he shot a remarkable 63 on Friday to move to 15 under for the tournament.
"I've known him all my years, and he's always been a guy who you could see him visualising the shot before he goes in, he takes his time".
"I didn't quite swing it right today, and I didn't quite have the shape, ball flight", said the 14-times major champion. At the age of 9, Tesori played at TPC Sawgrass before it opened to the public, hitting balls from the dirt of No. 17 with his grandfather and the first course superintendent. Woods finished on minus eight, one behind clubhouse leader Jason Duffner.
Woods and Spieth tied for the low round of the day on Saturday and gave themselves a "if I shoot an historic 60 on Sunday" chance in the final round. "Not to be negative, but I think Martin Kaymer had a 10-shot lead in Abu Dhabi and didn't win [in 2015], so you can never be completely comfortable - especially on this course".
I had questions going into Saturday after Simpson built his lead mostly with his putter and short game. "I tried to smash a sand wedge and blocked it a little bit".