The two will be paired together in the final group at Augusta National on Sunday.
This is not match play as it was for their singles match in Ryder Cup at Hazeltine.
"Really excited. Any time you can come out and have a lead on Sunday, you're always going to be excited about it", Reed told CBS Sports shortly after his round.
"A shot like that makes it fun", Mickelson said, but it was the exception in his 2-over 74 for a 7-over total of 223. They played in 3½ hours, about two hours faster than threesomes have averaged this week, and Casey called it "the best experience I've ever had with a marker". Instead, players enjoyed softer fairways with greens that continued to run firm and true thanks to the sub-air system installed in the course.
Three-times victor Phil Mickelson was also unable to gain any ground as he followed an opening triple-bogey with a bogey at the second en route to a disappointing two-over par 74 that left him at seven over on the week.
No sooner had McIlroy joined him atop the leaderboard at 9-under than Reed rolled in a nine-foot birdie at the eighth and a 25-footer at the ninth.
"I feel like all the pressure is on him", McIlroy said. He cooled until the end of his round, when a pair of birdies over the last two holes gave him a 65 and left him five shots behind. Now there is one day to go and I hope I can continue doing what I've been doing. "All the pressure's on him".
Anyone who walks along the 10th hole at Augusta National Golf Club has only to go a few steps and look left to consider something awesome. He went to August State, he's got a lot of support here.
Fowler also played his way firmly into contention for his first major victory.
Rickie Fowler made eagle on the par-5 second and was 5 under through eight holes.
Masters.com offers much of the same live coverage, too. Survive the stretch at Amen Corner, and holes 13-16 present plenty of birdie and eagle opportunities down the stretch. Since then, he has played his past 27 holes in 11 under.
"It was a great round of golf, I'm proud of how I handled it out there". "Yesterday on the back nine it kind of switched".
Reed attended the University of Georgia when he was just 17, but got kicked off the team after allegedly trying to hide an alcohol-related arrest.
That's when it hit the flag dead center and fell in for an eagle-3. "If you're going to shoot low, today's the day to do it".
KEY STATISTIC: Reed has rounds of 69-66-67. It's not an easy target, but players who can hit a high ball and land it softly may enjoy some success. Then he shot 79 on Saturday, falling 30 spots after failing to record a birdie despite the soft greens. Poulter said as they walked up the 18th fairway, he said to Woods, "It's good to see you healthy again". So for me, it will be same as I did last night.
Even without them, the show is just getting started.
So who is Patrick Reed? Still in the tournament but as participant not player. "I didn't know what my position was, but I think I'm in", he said after his round. Obviously he's playing well. "I just haven't gotten it done".
Reed produced a stunning reply with three straight gains between the eighth and 10th and, as McIlroy's round became somewhat scratchy on the back nine - bar a clutch birdie at 15 - the American's juggernaut largely continued to roll.
"Now I'm in the final group and I have a chance to win another major". Kessler (Karain, Justine's brother and Reed's caddy) knows that if I don't want 3-wood, he should pull 3-iron or something, never driver. Dustin Johnson, Justin Thomas and Justin Spieth - world numbers one, two and four respectively - will need a minor miracle tomorrow after struggling to stick the pace. Tiger Woods shot an even par 72 and is 4 over.
Woods bogeyed the first hole. Fleetwood is competing in his second Masters.
Danny Willett, the victor in 2016, followed an opening 75 with a 76 to miss by two shots.