Spieth won the Masters from start to finish in 2015 and was set to do it again the following year, only to blow a five-shot lead with nine to play to finish second.
The 2014 Open champion started with a birdie to quickly get back to level par, and he got his score into the red numbers for the first time when he made a three at the 3rd. South African Charl Schwartzel, a former Masters victor, also is 4-under.
That duo are two shots ahead of England's Ian Poulter, who had a 67 on his return to the venue where he came runner-up in 2008, and the American Justin Thomas.
She threw high winds and torrential rain across the Royal Birkdale course but the two-time major champion took it all in his stride.
The wind, blowing in the opposite direction to Thursday and gusting more than 30 m.p.h., proved a huge challenge.
Spieth, third in the rankings and twice a major victor, will tee off alongside Swedish defending champion Henrik Stenson. "I felt like I was in control of the golf ball pretty much all day". I just have to try to keep that going for the next two days.
As with Koepka, the last time Spieth teed it up, he won - at the Travelers Championship the week after the U.S. Open, so he felt like he was entering the week with good vibrations.
Consider this: The U.S. Open ended on June 18.
"I just don't mind grinding", Johnson said. One thing that didn't happen was golf. I've done the same thing for years, so it shouldn't take too long to get back into it.
Like the way Koepka capitalized at Erin Hills last month.
"I feel like this place really suits my game because you need a lot of imagination and have to play a variety of different shots". "After taking four weeks off, it's kind of nice to get back inside the ropes and finally get those competitive juices flowing".
Koepka considers himself the consummate gamer, though. After coming up short two years ago, Spieth is the headliner and the favorite to win at Royal Birkdale. "I'm very excited for tomorrow".
McIlroy has a lot more ground to make up, five strokes behind Spieth for the championship and just one better on the day, but given his recent performances and the streak of missed cuts, even getting to the weekend would have been worth commending. "I played really solid". I wanted to have those juices flowing. He thought back to 2011 at Royal St. George's, where McIlroy went 74-73 on the weekend.
Kuchar had it going in a big way in the afternoon, making the turn in that magical 29.
There were two critical junctures for Spieth on the back nine.
"I watched some of the golf [in the] morning on TV and it looked awfully challenging, " Kuchar said. "It seemed like the later your tee time the better the draw you got".
Despite those late bogeys, Kuchar can be quite secure that he will remain near the top of the leaderboard at the conclusion of the second round.
"I think I'm in a position where it can be very advantageous, just everything I've gone through - the good, the bad and everything in the middle", he said.
On the par-5 15, he missed the fairway left from the tee but he was still able to play an incredible low-cutting fairway wood that beat a path through the wind and rain onto the green and 20 feet from the flag. "It was in one of those rake marks, and my caddie told me to get inside 10 feet - that would be pretty good".