The Englishman is making just his second Rolex Series appearance at The Renaissance Club but looked very much at home as he opened up a one shot advantage over reigning Race to Dubai champion Lee Westwood and World Number Three Justin Thomas.
World Number One Jon Rahm was then at five under, fresh off his maiden Major Championship victory at the U.S. Open Championship, alongside fellow Spaniard Alvaro Quiros, Ryder Cup stars Matt Fitzpatrick, Tommy Fleetwood and Ian Poulter, another Englishman in David Horsey, Belgian Thomas Detry, South African George Coetzee and American Ryan Palmer.
Glorious conditions greeted the strongest field in the event's history in the second Rolex Series event on the 2021 Race to Dubai, and heavy recent rain had softened up the links layout.
There were four of the top five in the Official World Golf Ranking on show in North Berwick but it was World Number 353 Senior who stole the show as he moved to seven under.
The 32-year-old has two wins on the European Challenge Tour but just two top tens in 61 appearances on the European Tour, although he revealed he felt perfectly comfortable sitting above Major Champions and Ryder Cup stars.
"I've won at every single level I've played at," he said. "I think that's something that I've always been good at.
"I can get across the line and I feel comfortable when I'm at the top of the leaderboard. I don't shy away from the top of the leaderboard and when I'm out in front, I don't tend to back away.
"I tend to try to keep the foot on the pedal and I'm going to have to do that this week because there's going to be very little wind over the next three days and you're going to have to keep hitting good shots and keep rolling the putts in.
"It's a great field but, at the end of the day, you're still out there playing the golf course. Doesn't matter how good the field is or anything like that. All you can do is control what you're doing and try and do your best on the golf course and not particularly worried about the field.
"I mean, if you consistently hit golf shots out there and try to perform to the best of your ability, I'm sure that might stand me in good stead."
I feel comfortable when I'm at the top of the leaderboard. I don't shy away from the top of the leaderboard and when I'm out in front, I don't tend to back away
Westwood knows The Renaissance Club, having been made an honorary member when he lived in Edinburgh, and he made a fast start from the tenth, finding the green in two on his par five opening hole.
He holed a 25 footer on the 13th, recovered from a poor tee shot on the par five 16th and holed a 15 footer at the 18th to move up the leaderboard.
A run of six pars would follow but a stunning bunker shot on the seventh and tee shot to ten feet on the ninth meant he was the man to catch.
Senior also started on the back nine and he too hit the tenth in two before holing a 16 footer on the 14th and putting his tee shot to six feet on the 17th to turn in 32.
Putts of around 15 feet on the first and fourth had him within touching distance and he hit a smart approach into the fifth before making it a hat-trick from 18 feet on the sixth to take the lead.
A putt from similar range handed him a two shot advantage but a bogey on the ninth after missing the green left the door ajar for the afternoon starters.
Thomas proved to be the best of those as made two putts from just off the green at the par five third and then pitched to six feet at the fifth.
An incredible 90 foot eagle putt on the seventh may have been the shot of the day and he then made another two putt from just off the green at the 16th and put his approach to eight feet on the last.
Fitzpatrick and Poulter were both bogey free in their opening rounds, with Palmer getting to within one of the lead after ten holes before bogeying the 15th.
World Number Five Xander Schauffele, Ryder Cup Captain Padraig Harrington and home favourites Grant Forrest, David Law, Richie Ramsay and Connor Syme were in a group of 14 players three shots off the lead.