Senior led the strongest field in the event’s history in the second Rolex Series event on the 2021 Race to Dubai after the opening round but was soon overhauled by World Number One Rahm as he posted the early midway target of 11 under par.
Both Shubhankar Sharma and Gaganjeet Bhullar missed the cut.
Rahm was joined at the summit by Detry after he matched the Spaniard’s second round of 65 but Senior ensured he belonged at the top table with a composed four under 67 for a three way tie for the lead.
Reigning U.S. Open Championship winner Rahm carded eight birdies and two dropped shots as he improved his bid to reclaim his crown as king of the European Tour’s blue riband events.
Rahm has four Rolex Series wins – level with Tyrrell Hatton – and looked to have done enough to keep his nose in front as he prepared for Moving Day but Belgium’s Detry rolled in six gains in a flawless Friday to boost his chances of a maiden European Tour title.
“You can’t pretend to play 72 holes perfect. I would have taken six under before the day started,” Rahm said.
“We had beautiful conditions, we played the front nine without any wind. The second nine, me, Rory (McIlroy) and JT (Justin Thomas) struggled a little bit more, the wind picked up and that’s when it becomes a links golf course.
“Then again, links golf course, no wind, you can make some birdies.”
Senior has belied his World Number 335 ranking so far this week and came into only his second Rolex Series appearance with just two top tens in 61 appearances on the European Tour.
Instead, the Englishman will be heading a world class field at The Renaissance Club which has the likes of 2020 Race to Dubai champion Lee Westwood, former Rolex Series winner Matt Fitzpatrick, World Number Three Justin Thomas and Ian Poulter in the chasing pack.
“It’s just great to be back playing well. Great to be back holing some putts and it’s great to be back near the top of the leaderboard,” Senior said.
“I’m obviously excited. I don’t really show much emotion when I’m on the course or anything like that.
“As long as you can stay level and stay committed to what you’re going to do and see where it ends up at the end of the week, I’m not taking anything for granted.”
Rahm made a fast start with two birdies in his opening four holes to join Senior and Poulter, who also began with gains at the tenth and 11th, at seven under.
The Spaniard – starting on the back nine – added a third birdie at the 14th to take the outright lead before getting hot as he approached the turn.
A hat-trick of birdies from the 16th saw Rahm card a front nine of 29 to propel him to 11 under par, extending his lead at the top of the leaderboard to three strokes with Poulter his closest challenger.
He picked up his fourth straight stroke at the first to move to 12 under but wayward tee shots at the second and fourth yielded in bogeys to slip back to ten under alongside South African George Coetzee, who carded five birdies on his front nine.
Thomas applied pressure on playing partner Rahm by holing a birdie putt from six feet to sit at nine under before the Spaniard moved clear at the seventh.
Despite finding the rough with his tee shot, Rahm got up and down from 60 yards to roll in his eighth gain of the round to return to 11 under just as Coetzee recorded his second bogey of the tournament to drop two adrift.
Coeztee could not close the gap as birdie putts at the 14th, 15th and 16th pulled up inches short before he finally found the cup at the penultimate hole for a gain to sit one behind.
He narrowly missed the chance to join Rahm at the summit at the last, but the Spaniard was not alone for long.
Detry kickstarted his second round at the third when he got on the par five hole in two to pick up a shot before producing a stunning approach to four feet from the thick rough at the next for another gain.
He made it three birdies in a row as he dialled in his tee shot to four feet to get to eight under and he improved his score by one at the seventh with a seven foot putt.
The Belgian continued to create birdie chances but remained at nine under until the par five 16th, where he got on in two before two putting to reach double figures.
And just like buses, Detry only needed to wait until the next hole to card his sixth and final gain of the round when he rolled in from 10 feet.
“I believe that I’m doing all the right things,” Detry said. “I’ve been working hard with my coaches, as well, to work and improve under pressure and working with some mental people, as well.
“I think it’s part of the process and I’m pretty confident it will come. It’s all very nice and hopefully I can keep it going.”
Overnight leader Senior picked up where he left off from on Thursday with a birdie at the fifth to climb to eight under, only to hand back the shot immediately.
The 32-year-old was one foot away from an eagle at the seventh as he returned to eight under and picked up another shot at the 11th when he dialled into two feet.
Senior was inches from reaching ten under at the 14th but got there at the 15th after putting his approach to four feet to sit one shot adrift of the leaders with three holes to play.
He found the rough with his tee shot at the 16th but got up and down to eight feet and when he rolled in the birdie putt, he was co-leader before closing out with successive pars.
Westwood sat one shot back thanks a brilliant eagle-birdie-birdie finish, sitting with Fitzpatrick and Coetzee, who both carded six birdies and one bogey in their second rounds.
Poulter and Thomas lead the pack at eight under alongside Scotland’s David Law, who posted back to back 67s and Spaniard Alvaro Quiros, who mixed six gains with three dropped shots.