22 July, 2017
The two-time major victor, who won at the Travelers Championship last month, was particularly pleased with his composure and driving off the tee.
He told BBC Sport: "It is important to get in the red numbers".
As Spieth and Koepka went out, the wind had died down but torrential rain moving in from the Irish Sea created its own problems and waterlogged greens forced a brief halt to play around 5:30pm (1630 GMT).
"And you take the burnout factor and the injuries and just being fed up with it, the pressure of being Number 1 was hard, as well. So really nice to capitalize on it".
Spieth carded five birdies in a bogey-free effort, and he will be in full contention to scoop the major if his approach play remains so strong.
"I come in having forgot just how good it was".
Spieth shot 31 on the front nine before carding 34 on the back nine.
"Every time I come over here the ground is so hard", he said. The second shot landed in a location where I could pop it up and if it ended up going 15 feet past the hole then it was going to be a good shot.
He made a miraculous par save from left of the green at No. 10 and rolled in a mid-range putt for birdie at No. 14.
"Padraig won the thing from the bad side of the draw, so it can be done", McIlroy said. I was shocked by the way he stumbled early, and he could have packed it in on the back nine with a woe-is-me waltz to the clubhouse.
"It was very nerve-wracking on the first tee", said the 38-year-old.
The Merseyside golf fans did their best to try and inspire the star who learned the game playing just a few miles down the road at Southport and Ainsdale. If you can do that, you can get around here and shoot some good scores.
The American was not perturbed ahead of his arrival for the third major of the year despite not playing on a links golf course in the lead-up and said he enjoyed the challenge of using his imagination and creativity. That was really the only thing that at Muirfield (The Memorial), and at the U.S. Open, I mean it was really just putting. "And ultimately my career is going to be defined by how I do in these tournaments".
Justin Thomas and Ian Poulter, runner-up at Birkdale in 2008, are also among those at three-under, with Japan's World No 2 Hideki Matsuyama and two-time former Open champion Ernie Els in a group at two-under.
World Number 6 Jason Day is looking to re-capture the form he had at St Andrews during The Open Championship in 2015, a top-5 finish his best result at the prestigious tournament.
Casey shot a 66 along with Schwartzel, while Spain's Rafa Cabrera-Bello, victor at the Scottish Open last weekend, is three-under.
You just have to feel it.
