The Sheffield native, who only turned pro last year, shot a six-under 65 to leave his elder sibling in the shade after he had to be content with a 67.
Alex once carried Matthew's bag and watched on from beside the 18th green when he won the US Open at Brookline in 2022.
But on Saturday he grabbed centre stage with seven birdies in a superb round only bettered by the course record 63 posted by Masters champion Jon Rahm.
Not that there is sibling rivalry when it comes to who finishes higher this week.
"We're brothers at the end of the day as much as we're golfers," Alex, who competes on the second-tier Challenge Tour, told reporters.
"I root for him, he roots for me.
"If either shot level par, one-over, four-under, it doesn't matter - I would've still been proud of him. Obviously, very grateful he shot a good round today, and it'll be a nice dinner tonight I'm sure."
Unfortunately, they will not be playing together in the final round on Sunday as Alex is on four under with Matthew on two under. But Matthew said he will not mind if their parents Russell and Susan focus on Alex again.
"I told them to go focus on him. That's more important. This is my, what, ninth (Open), so they've seen all nine of those," Matthew, who won the silver medal as best amateur in 2013, told reporters.
"This is his first, so that's more important.
"I think they caught a few of my early holes and got back to watch him."
Alex only earned his first start in the Open earlier this month when he finished in a share of fourth place in the 36-hole final qualifying event at West Lancashire where there were five places up for grabs.
The former US college golfer at Wake Forest University, twice a member of Britain's Walker Cup team, is ranked 561 in the Golf World Rankings but looks destined to rise quickly if the last few days are anything to go by.
"I feel like when I play my own game and I play well, I feel like I can compete for the most part," he said.
"Today was a super special round. I'm not familiar with this environment and the amount of people out here, but me and my caddie had a great time out there and things went our way, which was super cool."
This year's Open has featured two sets of brothers competing with the Fitzpatricks joined by Danish twins Rasmus and Nicolai Hojgaard.
