Alex caddied for his older brother Matt when he won the US Amateur at Brookline in 2013 and watched from beside the 18th green as the two-time Ryder Cup player sealed his first major title on Sunday at the same venue.
"It's amazing," said Alex, who recently turned professional himself after a successful amateur career which saw him reach sixth in the world rankings.
"I actually flew home yesterday (Saturday) afternoon and then flew straight back when I saw he was tied for the lead. It's been a hectic 24 hours but it was definitely worth the 150 dollars coming back.
"I couldn't be prouder of him. It's his dedication to getting better each day. If you look at 99 per cent of the field, none of them would go through what he does to get better, even one per cent a day."
Fitzpatrick's parents Russell and Susan were also at Brookline to see their son's victory, just as they were nine years earlier.
"I can't believe he's won this at the same venue," Russell said. "It's a really special place. We've stayed with the same family, we love Boston. We're moving here next year!
"It's not just for Matt. I'm so happy for Billy (Foster). He's been 40 years as a caddie out there and always wanted to win a major championship. I'm so delighted he's got one as he's the best in the business."
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