SOUTHPORT, England — A former sixth-grade math teacher, Michael Greller is strong in numbers.

Jordan Spieth leaned more on his caddie’s words at the British Open.

“This is as much mine as it is his,” Spieth said as he clutched the silver claret jug. And then he turned to look over at Greller and said, “You deserve all the credit in the world for this major championship.”

Greller still had to work out some new math at the most critical juncture of the final round at Royal Birkdale. Spieth was hitting his third shot from the driving range and couldn’t see any part of the par-4 13th hole. He thought he was about 270 yards away and was planning to hit 3-wood. Greller estimated the yardage closer than that and told him to hit 3-iron.

“I certainly didn’t have any numbers from the right side of the range,” Greller said. “Honestly, getting a good line was tough. He thought it was a 270-yard shot. I was just looking at all that gorse and thought it was a little shorter. Short was fine. From there, he did what he has always done.”

In a situation like that — has there ever been a situation like that with a major on the line? — Spieth said he normally would trust his own instincts.

But not this time.

“On that one, he seemed very confident,” Spieth said. “He was very adamant about what club to hit, and it gave me the confidence to hit it, because sometimes when that happens I’ll still go with what I think. But he was right on.”

Those weren’t the only meaningful words on Sunday.

It was the calm in his voice when Spieth’s world was falling apart. It was reminding Spieth at the right time who he was and what he has accomplished, using as a reference a photo from a beach holiday in Mexico where Spieth posed with a group of star athletes that included Michael Phelps and Michael Jordan.

And it was reminding him that his bogey on the 13th, while costing Spieth the lead for the first time all weekend, was the shot of winners.

As they walked off the 13th green, Greller stopped Spieth.

“He said, ‘That’s a momentum shift right there.’ And he was dead on,” Spieth said. “And all I needed to do was believe that.”

How the former math teacher from the Seattle area stumbled into a job for golf’s hottest property remains an amazing tale.

It dates to more than a decade ago when Greller, who caddied in the summer for extra cash, noticed a player in U.S. Amateur Public Links who was carrying his own bag. He offered to caddie for Matt Savage, free of charge, and they made it into match play before losing in the semifinals.

Savage’s golf instructor in Louisville, Kentucky, was Mike Thomas, the head pro at Harmony Landing and father of a teenager, Justin Thomas. So when the younger Thomas played the U.S. Amateur at Chambers Bay, Savage hooked them up.

A year later, Thomas recommended Greller to his best friend in golf for the U.S. Junior Amateur — Spieth.

Spieth turned pro before Thomas and asked Greller to work for him. With some trepidation, Greller left his job as a math teacher to work for a 19-year-old who didn’t even have a tour card. That was four years, 11 PGA Tour victories and three major championships ago.

Also at Royal Birkdale was Jim “Bones” Mackay, perhaps golf’s most famous caddie after spending 25 years with Phil Mickelson. Before signing on with NBC Sports, it wasn’t clear if Mackay would work for another player. One of the more short-sided suggestions was that he work for Spieth.

It was offensive to Greller, laughable to anyone who has watched them.

The foundation of player-caddie success is the relationship, and Greller is rooted in that. It’s what attracted Spieth to him in the first place.

It was Greller who brought levity to a nervous situation when Spieth took a four-shot lead into the final round of the 2015 Masters by saying on the first tee, “Aren’t you glad you’re not at Pasatiempo right now?”

That’s where the Texas Longhorns were playing a college match. Spieth would have been a senior. Instead, he won a green jacket.

Greller lived through Spieth’s collapse at Augusta National last year. And he found the right tone, the right words, when Spieth was headed for another one at Royal Birkdale with three bogeys in the opening four holes to lose the lead.

Most memorable for Spieth was when Greller brought up the photo from Cabo San Lucas.

“We walked off 7 tee box and he made me come back,” Spieth said. “He said, ‘I’ve got something to say to you.’ He said: ‘Do you remember that group you were with? You’re that caliber of an athlete. But I need you to believe that right now because you’re in a great position in this tournament.’

“It was just the right time,” Spieth added. “Just his belief, when I know him so well, fed over a bit. And all I needed was just a little bit of self-belief to be able to produce what I had there.”

A bogey from the driving range. A birdie-eagle-birdie-birdie for an encore. A claret jug with room for only one name.