The past seven majors have been won by first-time winners, although Royal Birkdale has a quirky habit of rewarding former champions and consecutive winners.
Spieth threw his hands in the air, shook his head and called over the match referee. They had their hiccups on Friday, but with calmer conditions favoring low scores, there was a steady approach by both golfers.
One player in each of the first three groups also went out of bounds and the first eight groups were a combined 23 over par for the opening hole before Sweden's Alex Noren recorded the first birdie.
McIlroy battled hard and two-putted the 15th and 17th for birdies, and the packed grandstands around the 18th green gave him a generous ovation when he knocked in another from 15 feet to revive his hopes of a first major win since the 2014 PGA Championship.
The American shot a 69 on Friday in miserable weather conditions to sit at six under par and lead the championship by two strokes from his compatriot Matt Kuchar.
Spieth now enters Sunday with a chance to become the second-youngest player, after Jack Nicklaus, to win the first three legs of the career Grand Slam.
Harrington, the Open champion from 2008 at Birkdale, had one bogey and one double bogey.
"It's tough to make putts when you're not that close to the hole".
"I lined up way left with the wind whipping off the left", said Spieth.
"I don't know which is going to be harder, winning the Claret Jug again or going skydiving afterward because that thought scares me a little bit", the deadpan Swede said Tuesday.
But yesterday leading British player Ian Poulter said the fans on the course were the real champions.
He arrived on the tee in a buoyant mood after eventually firing an opening round of 71, making four birdies on his back nine, and he took his good form into round two. Kuchar lagged his eagle putt to tap-in range to reach 9 under and momentarily join Spieth in the lead, but only until Spieth poured in the tricky birdie putt.
Spieth was consistently excellent, hitting all but three greens - and putting from off the green on one of those - and shooting a 31 and 34. Just like that, he was three shots behind. "Certainly, I'm not out of it".
"But I'll stay loose, smile a lot, and that's what I did today, and if I play the way I've been playing things will take care of themselves. I'm very excited for tomorrow". It was the best single round in major tournament history. Kuchar, playing in the same final group, shot a 66. So the course played completely differently.
The Open Championship at its mid-point is all about Jordan Spieth.
The forecast for Thursday is mostly fine with winds gusting to 20-25 miles per hour in the afternoon, while Friday will be breezy in the morning with outbreaks of rain in the afternoon.