The Holywood star looked certain to trail defending champion Jon Rahm by four shots after the Spaniard got to five-under with one to play.
But the big Basque made a double-bogey six at the last, driving into sand and taking two to get out to card a 71 that left him solo third on three-under.
Attitude has been key for the four-time Major winner in Boston and he needed to show it in spades at a cool and breezy Brookline last night as he survived a helter-skelter afternoon to keep alive his hopes of ending his eight-year winless streak in the Majors.
The Holywood star has prided himself of staying on an even keel this week, reminding himself not to panic when things go wrong.
And he needed that poise in an attritional third round when the temperature plummeted and the wind switched to the north, making this classic test a fearsome examination with just seven players breaking par on the day and only nine in the red going into the final round.
“Yeah, it was one of the toughest days on a golf course I've had in a long time,” McIlroy said after getting up and down for crucial pars at the 13th, 15th, 16th and 17th before making par at the last after hooking left into the grandstand before getting a drop that allowed him to find the green and escape with a par.
“I just needed to grind it out, and I did on the back nine,” said McIlroy who hit only six fairways. “You know, to play that back nine at even par today was a really good effort, I thought. Just kept myself in the tournament. That's all I was trying to do. Just keep hanging around. I felt like I did well to get it in under par for the tournament at the end of the day.
“I holed some really good putts for pars coming in. 13, 15, great up-and-down on 16, good putt on 17. Then was really fortunate at the last to get that drop from the grandstand and be able to hit it on the green from there. It wasn't the greatest of starts, but sort of steadied the ship a little bit and happy to get it in a few lower than it could have been.”
The Holywood star was three-over after a ragged and nervous display over the first six holes when no aspect of his game was working and the magical putting touch of the first two days abandoned him as he bogeyed the second, third and sixth and missed his few birdie chances.
He turned for home five adrift of Masters champion and world No 1 Scottie Scheffler, who screwed a 101-yard wedge shot back into the hole for an eagle three at the eighth to go to three-under for the day and six-under for the championship and two clear.
But within 90 minutes on a snakes and ladders day, the world No 1 was three shots behind Zalatoris, who set the clubhouse target at four-under with a brilliant 67 as he racked up a double bogey five at the short 11th, then bogeyed the next three holes before he birdied the 17th and saved par from 15 feet at the last to card a 71 that left him tied with Keegan Bradley (69) and Adam Hadwin (70) just three shots behind Rahm on two-under.
English star Fitzpatrick carded a 68 as his newly acquired length paid off again.
Zalatoris and Fitzpatrick were tied for second behind Chile’s Mito Pereira heading into the final round of the PGA Championship in Tulsa last month. But the American would lose in a playoff to Justin Thomas as the English star faded to tied fifth after a 73.
“I think especially coming off the PGA it gave me a lot of belief and confidence that I belong in this situation,” said Zalatoris, who has two runner-up finishes among his five top-10s in seven major starts. “I've put myself in this situation a few times in my career, and obviously have to go out and get it tomorrow.”
Fitzpatrick, who won the US Amateur at Brookline in 2013, has gone from 59th to 10th for strokes gained off the tee over the past few seasons thanks to speed training technique know as The Stack.
“I think I didn't have my A-game that day, and still just missed the playoff by one or two shots,” Fitzpatrick said of Tulsa. “I can't even remember now. I think that was the thing is that I felt like if I played anything like my A-game, B-game, whatever, I would have been in even better contention.
“For me I think taking that away from it, knowing that I can play better than I did and where I finished definitely brought into this week as well as, obviously, having great experiences here before.”
Zalatoris and Fitzpatrick were briefly overtaken by Rahm, who was two-over for the day through 13 holes but birdied the 14th, 15th and 17th to get to five-under before his late mistake.
“I'm very content. I'm not going to lie,” Rahm said. “It's infuriating in a sense to finish that way with how good I played those holes, but like I kept telling myself, if on the 14th day you tell me you can post one-over par and not play the last five holes, I would have ran to the clubhouse because of how difficult it was playing. I would have taken it, no questions asked. I think I have to consider that. I have 18 holes, and I'm only one shot back. That's the important thing.”
McIlroy will not catch the pace-setters if he plays as he did early yesterday.
He hooked his tee shot so badly at the first, he finished in the hospitality tents and was off with his distance control at the second, third and sixth, dropping shots each time.
After missing good birdie chances inside 10 feet at the seventh and eighth, he found himself five shots behind Scheffler and outside the top 10 before clawing his way back into contention.
After making a 12 footer for a two at the 11th, he bogeyed the 12th, then hooked into trees at the 13th but got up and down from 170 yards for, rolling in a 15 footer to remain four back before getting up and down from sand at the 15th and 16th and then making a 12 footer for another par at the 17th to share seventh on one-under.
It was his eighth single putt in the last nine holes, displaying the kind of grit required to end that long wait today.
“That was all Harry,” he said of that crucial up and down at the 13th. “I was ready to hit a 4-iron and try to play it off my front foot and hit it up over those trees. He said to me, look, you could pull it off, but you could also make a 7 or an 8 doing it. He goes, why don't you play it out here, and we can try to get it up-and-down. That was huge.
“That's the benefit of having a good caddie and someone that's strong and -- he said to me, look, just go here. Don't even think about it. For once I listened, and it paid off, and I was able to make a good par.”
The world number three knows it could have got away from him but he was thrilled to hold it together.
“It all evened out at the end of the day, but I'm sort of going home thinking that I held this round together when it could have got away from me quickly,” he said.
Pleased he’s now got the putting game to keep in touch when he’s struggling, he knows it will take some outstanding golf on Sunday to win.
“I mean, 67 from Will out there today is unbelievable,” he said. “Such a good score. 68 from Fitz as well. I think depending on the conditions tomorrow, that's what it's going to take. From me it's going to take something like that to get the job done, but as we've seen today, things can change so quickly.
“I certainly thought I was going to be a few shots further back than I was at the end of the day, but Jon struggled there coming in. Even though it was such a tough day and I feel like I battled well and whatever, to still only be three back going into tomorrow I feel is a good thing for me.”
Tee times (all Irish times)
1912 Rory McIlroy, Sam Burns
1923 Scottie Scheffler, Adam Hadwin
1934 Keegan Bradley, Jon Rahm
1945 Matt Fitzpatrick, Will Zalatoris