Jordan Spieth resisted a spirited challenge from his fellow American Boo Weekley to preserve his one-shot lead in the Travelers Championship in Cromwell, Connecticut.
The former world number one has been the clubhouse leader since Thursday but needed a birdie on the 18th hole of his third round to pull a stroke clear of Weekley, who signed for a five-under 65.
England’s Paul Casey will be in title contention on the final day after climbing into a share of fourth place with CT Pan and David Lingmerth following a 66 that contained five birdies and a bogey.
McIlroy managed to make par at one point after hitting a tee shot into a water hazard, and he later told Golf Channel: “I would have taken a par after the tee shot. Could have made birdie, but didn’t.”
Spieth recovered from his early double bogey with three birdies across his next eight holes and afterwards confessed it was his putting that saved his round.
After taking only 29 putts to finish on Friday, the Texan said: “I wouldn’t say I played my B-game. I think I’ve been up there playing pretty solid golf.
“It’s just, hitting more greens. The ones I missed, I didn’t really need to miss. I was trying to do too much. That’s kind of what gets me in trouble on a course like this.”
Johnson Wagner and Brett Stegmaier were a stroke behind Spieth after day one but both Americans followed 64s with 72s to slip out of strong positions.
That left Merritt and Reed as the closest title challengers on seven under while Patrick Rodgers was one of the day’s biggest movers, his five-under 65 boosting him to within three strokes of the lead.
Ireland’s Padraig Harrington is among those tied for eighth with Rodgers after a one-under 69 while Paul Casey carded another 68 to retain an outside chance of climbing the ladder in Cromwell.
Anirban Lahiri, meanwhile, revealed how a recent stint at a meditation centre helped him shoot 63 to fly 108 places up the leaderboard.
The Indian said: “You go to the centre for 10 days, you switch your phones off. There is no TV, you’re not allowed to read.
“You’re meditating. You’re not allowed to talk. So there is like complete silence for nine and a half days or so, and you’re meditating for 10 to 12 hours a day.
“I find it very healing for me, and a lot of people who have gone there benefited from it. It’s not something I do specifically for my golf, but it’s something that’s helped me.”
Irishman Padraig Harrington is five behind Spieth, the 2015 Masters winner, after shooting five bogeys in a one-over 71 that leaves him four under par.
McIlroy - like Spieth making his tournament debut here - is 12 strokes off the lead and all but out of the race after following up Friday’s poor 73 with an even-par 70.
Also suffering frustration was Charley Hoffman, who at one point took over at the top with four birdies across the front nine. On his way back in, however, he at one point shot 5-5-5 to drift away to a 68.
Spieth, 23, will be joined in the final pairing by Weekley after the Floridian found birdies on the 11th, 13th, 15th, 17th and then 18th holes.
Weekley, 43, said of his young opponent: “Jordan’s a good dude. He’s one of the most humble kids out here that play with us right now. Day in and day out, he’s one of the most humble kids.
“My hat is off to him, and for who he is as a person.”
Spieth, who is chasing his 10th PGA Tour title, replied on ESPN.co.uk: “I hope I’m having as much fun each day as Boo’s having. I mean, he lives it up. It’s fun to be around Boo.
“He’s always very nice to everybody he sees. He’s a guy everybody very much respects and really likes being around, and that’s fun.”