The evidence came early in his first round during The Open Championship at Royal Birkdale on Thursday: Jordan Spieth was locked in. Spieth shot a 5-under 65 to take and hang on to an early co-lead with U.S. Open champion Brooks Koepka at the 146th Open.

After making par at the first, Spieth pull-hooked his drive into some bumpy gorse off the second tee. From there, he hit an off-balanced approach that landed about 12 feet from the cup. He poured that in for birdie and never looked back.

It's not that Spieth makes par from dead areas when he needs to save the momentum of his round, it's that he makes birdies from those spots and actually accelerates the momentum. He made five birds on Thursday on a day when he hit just five of 14 fairways. It was as Spieth-y as a round gets.

This does not diverge from the trajectory Spieth has been on all year. He has not driven the ball particularly well all season. What he has done is hit tremendous approach shots (remember, he's No. 1 in strokes gained on approaches on the PGA Tour), and that served him well on a second-shot course like Birkdale.

This is, remarkably, Spieth's first opening round under par in a major since the first round of the 2016 Masters, and it's the second-lowest first-round score ever at an Open at Birkdale.

"The first guys off got some really tough conditions," Spieth told Golf Channel's Jimmy Roberts. "By the time we were teeing off, it was just 15 miles-per-hour wind and a little chilly. Nothing like we're going to experience tomorrow. 

"It's a gettable golf course if you're controlling your ball off the tee. You're able to hit the ball in the middle of green,s and they aren't too severe so you actually have birdie putts from the center of greens even if you're out of position. Tried to keep our head down today. Stole one at No. 2 and started to get it going around the turn."

It's important to note here that even though Spieth only hit five fairways, he was rarely truly in trouble off the tee. He was in spots just a few paces off the fairway where it was easy to hit greens in regulation (which he did 15 out of 18 times). 

To stay bogey-free, though, was mighty impressive. Nobody shot a bogey-free round the last time the Open was here in 2008. Now, Spieth will likely co-lead going into the final 54 holes alongside Koepka.

"I've experienced being an underdog and the favorite so many times in my short career that I don't really think much of it at this point," Spieth told Golf Channel. "Today was a really good day. It was a really good start. Tomorrow is going to be a very different type of day if what's forecasted comes in. 

"We've expereinced that the last couple years. We know a little bit what to expect which could bode well for us ... Gotta keep your head down. Birkdale has yielded 3 over as a winning score. We don't know what we're going to get the next few days. There's a good chance where I'm at right now is plenty good enough right now so par becomes a very good score."

The winning score at this course in both 1998 and 2008 was even par or worse. The problem now for the field is that Spieth is elite with a lead, and he's even better when all he has to do is protect it. If Spieth was making birdies from all over the yard on Thursday, what's going to happen to the rest of this field when all he has to do is make pars? 

Yes, he'll have a galloping mate in Koepka, but it's not going to be super crowded atop those blue and yellow leaderboards.

This tournament is far from over, of course, but the formula this week (pending the weather) would seem to be something along the lines of race out in front on Thursday and protect coming home. Spieth accomplished the tougher task on Thursday, and now he gets to defend at the top for rest of the week. That's going to make for some great drama with Koepka trying to pull off the double Open, everyone else trying to run them down and Spieth trying to touch off the third leg of the career Grand Slam.

Game on in Southport. Spieth is here for the duration.