SANDWICH, England — Jordan Spieth, a born communicator, was, as usual, talking to himself, to his caddie, to his ball in flight and on the roll.There was no shortage of good news to discuss on Thursday as Spieth dodged the danger and rolled in the birdies on the first day of the British Open.It was Spieth’s first time playing Royal St. George’s, and you would never have known as he finished with a five-under-par 65, one shot off the lead held by Louis Oosthuizen.“For the most part historically I’ve come into venues I’ve never seen before in any tournament, not just an Open, and I’ve always tried to find something I love about it,” Spieth said. “There’s been times recently where I’ve said, ‘Man, I just really don’t like this place.’ But I came in here, and I’ve been in a really good mood about it.”Local knowledge is not necessarily power in golf, particularly when the wind picks up significantly in the afternoon.Phil Mickelson finished in a tie for second, with Dustin Johnson, the last time the Open championship was held at Royal St. George’s, in 2011. Less than two months ago, he won the P.G.A. Championship at age 50, becoming the oldest golfer to win a major championship.But none of that carried over on Thursday as Mickelson, now 51, misjudged chips, missed putts and put far too many of his tee shots into rough that is particularly thick this year.“Fore! Right!” he shouted on the ninth tee after his latest errant drive.“That’s the one I don’t understand,” Mickelson said to Tim Mickelson, his brother and caddie, as he walked glumly toward his ball.“I thought if I could get a little closer, it wouldn’t do that,” he said of his stance.“It wasn’t too fast was it?” he asked, referring to his swing tempo.The doubt was not just audible. It was visible. After finishing the front nine with yet another bogey, he stood next to the ninth green, arms folded and head down.“Come on Phil!” shouted an English fan, one of the approximately 32,000 spectators on the course on Thursday.“Cheer up Phil!” shouted another.But there was no joy ahead as Mickelson pushed on with the increasingly strong wind rattling the pins and making ripples and waves in the long fescue grass as if it were the surface of an inland sea.He finished at 10 over par with an 80, tied with Deyen Lawson for the worst score of the day in the 156-player field. It was Mickelson’s worst opening-round score in any major and his second-worst score at the British Open, behind only the 15-over 85 he shot in the third round in 1998 at Royal Birkdale.“I didn’t see that coming,” he said after leaving the scorer’s tent and declining — politely — to discuss the round.“I’m in last place,” he said, avoiding eye contact. “It’s just mocking me.”Told that was not the objective of the interview, he continued walking.“Well, I’m doing the best I can,” he said. “I’m sorry. I’ve got to play early tomorrow.”It was the latest reminder of how quickly form and conditions can shift, particularly on a links course. It was also a reminder of what a cerebral game golf remains.Spieth knows it too well. He looked ready for a long run at the top of the game when he broke through to win 10 tournaments over three seasons, becoming the second youngest Masters champion — behind Tiger Woods — in 2015 and reaching No. 1 in the world rankings. He won the British Open at Royal Birkdale in 2017.He could not sustain that momentum and dropped to No. 82 in the rankings at the end of 2020, but he has been resurgent this season, recording eight top-10 finishes, tying for third at the Masters and winning the Valero Texas Open.“Golf is a game played between the ears, right?” he said on Thursday. “When it’s not going great, you can certainly lose quite a bit of confidence in it. That was the first time I’ve had to really try and build confidence back up, and it takes time. It’s a combination of obviously getting things figured out mechanically but also then putting it to the test and mentally stepping up with enough oomph to go ahead and pull off some shots. That’s how you build the confidence is using that improvement I think physically on the course under pressure. By no means do I feel like I’m where I want to be mechanically yet, but this year has been a really, really good progression for me.”At age 27, Spieth is back up to No. 23 in the rankings and rising, although he may have to face different conditions when he plays later in the day on Friday.The top three golfers on the leader board all started Thursday morning: Oosthuizen, Spieth and the American Brian Harman, who also shot a 65. At that stage, the fairways and greens were softer and more receptive, but not everyone thrived.Bryson DeChambeau, one of Spieth’s playing companions, tried to deploy his prodigious length off the tee to his advantage but repeatedly found the rough. He hit only four of 14 fairways and expressed discontent with his driver rather than his tactics. He finished with five bogeys and four birdies and a 1-over 71.“I think No. 1 on this golf course, it’s hit the fairway,” Oosthuizen said. “You’re not going to be able to do much from the rough here or the fairway bunkers. If you aren’t comfortable with a driver around this golf course, then don’t be scared laying further back, as long as you can get in the fairway.”Spieth, not one of the tour’s long hitters, would surely agree with that game plan after his opening round at Royal St. George’s.“I kind of got away with a couple tee shots in the first cut that maybe if it was firmer may have worked their way just into the fescue,” he said of the Open’s wispy rough. “It’s a course where you have so much undulation in the fairways that if it gets firmer it gets very bounce dependent.”On Thursday, he managed the conditions with aplomb, making four straight birdies on holes No. 5, 6, 7 and 8 and two more on 15 and 16.Mickelson could not conjure a birdie at all despite the roars of support from fans gathered on the knolls and behind the ropes.On No. 18, after slicing his final drive over the metal fencing that separated the gallery from the fairway, he had to hit his second shot from the rough near the first tee, the place where his nightmarish round had begun a few hours earlier.“You’re the champ, Phil,” shouted a fan, using an unprintable adjective before champ, as he trudged toward the green.“PGAAAAAA” shouted another, dragging out the vowel.Mickelson flashed a discreet thumbs-up and nodded but could not avoid a final double bogey.