The American world number three shot 69 to follow an opening 65 and finished on six under par, two strokes ahead of compatriot Matt Kuchar, as his bid for a third major title gathered momentum. You just don't know where your breaks fall and where they don't.
"To be in after two days and be under-par for this championship after the way I started, I'm ecstatic with that", said McIlroy.
"Massive", he said about the chip-in par.
On the 10th, Spieth - black puffy vest over grey, longsleeve shirt - pulled his second shot onto a grassy bank left of the green.
Austin Connelly, who grew up in Dallas and shares a swing coach with Spieth, extended his remarkable run with birdies on his last two holes for a 66. Both players sit on three under par. His game plan proved to be correct, at least in the opening round. It is a false narrative that dogs Spieth, one that could be viewed as an insult to his competitive resolve, his ability to fight through adverse conditions, the unrelenting desire to simply will the golf ball into the hole. But McIlroy made a double bogey on the 10th hole and was at 1 under. After hitting his tee shot into a fairway bunker on 10, he blasted from the sand and hit his third shot on the par-4 through the right edge of the green. The two-time major victor chipped in for par at 10 before back-to-back birdies at 11 and 12 when most of the field were struggling just to make pars.
On the most favorable day for major scoring in recent memory, a day that saw a record 62, Jordan Spieth saw the onrushing horde of would-be challengers for the Open Championship and merely threw down a bogey-free 65, capped with an 18th-hole birdie, that left no doubt about who's in charge this week.
Breaking par can't be considered a formality but if Spieth can navigate the first six holes without alarm, he'll have one hand on the Claret Jug.
Grace's unprecedented round featured five front-nine birdies and he was seven under for the day with two to play after draining lengthy putts for further gains at the 14th and 16th.
T5. Hideki Matsuyama (-4): The list of players to take seriously as a threat to Spieth or Kuchar is short, but Matusyama is absolutely on that list.
And just when it seemed like he too would fall victim to the near gale force winds and driving rain, he stemmed the flow of dropped shots with a touch of genius. I got away with one there.
But Spieth is where we got used to seeing him.
To that end the 39-year-old, who has yet to win a major and has a best Open finish of ninth in 2012, says he never approaches a round with a gameplan but makes up his mind what to hit and where to hit it when the gets to a hole.
McIlroy remains in hot pursuit after continuing his spectacular fightback with a superb two-under 68. And it's tough not to like him now.
T7. Dustin Johnson (-3): You have to put D.J. on that list of long-shots as well, but he'll need another 64 or a 63 to get up into a championship-winning score after going 71-72 before the weekend. It creates a bit of a challenge and I felt I did a good job of it.
"Good time to back me, I think", laughed the Northern Irishman, who didn't cope well with strong winds at the 2011 Open and said at the time, "These conditions, I don't enjoy playing in really".