McIlroy (12 under) lit up Augusta National early on Saturday before his American nemesis answered in the shadows, nailing a 50-foot birdie putt on the 18th to sit just two shots back.
It means McIlroy will need to exorcise new and old major demons, the Northern Irishman in a final-round group pairing with the imposing American who broke his heart at Pinehurst to snatch the US Open last year.
But McIlroy has already shown his resilience this week and on Saturday was ruthless, a historic birdie-eagle-birdie start the first occasion of a player starting a round with three straight threes in Masters history.
His chip in on the second electrified the huge gallery and another eagle on the 15th offset two bogeys in his second consecutive round of six-under 66.
DeChambeau (69, 10 under) fired up with back-to-back birdies down the stretch - and again on the 18th - after he had faded to seven-under-par through 14 holes.
Canada's Corey Conners (70, eight under) has been in the mix all week while former winner Patrick Reed surged into the reckoning with Sweden's Ludvig Aberg (both shooting 69s and six under).
Australian Jason Day (71, five under) continued his steady campaign that again on Saturday was frustratingly unable to hit top gear.
He was six under through the turn and had putts to move within two shots of leader McIlroy.
He chipped in on the 14th but then, after finding the middle of the sloping 16th green, opted to chip in fear of his putt sliding into the water.
That effort left him with a testing par putt, Day missing that then almost holing an approach to the 18th after spraying his drive to finish with a par.
McIlroy has been chasing his fifth major and first Masters title to complete the set since 2014, victory on Sunday would end a 25-year wait for golf's next grand slam champion since Tiger Woods in 2000.
The 35-year-old looked to have scuppered his chances in the first round when he chipped from close-range through the green and into the water and made the first of two sickening double-bogeys.
He rallied superbly with a second-round 66 to sit just off 36-hole leader Justin Rose (five under) and by the fifth hole on Saturday was 11 under and four shots clear of the field.
An eagle at the par-five 15th, where he made a seven in the first round, pushed the him clear again.
He'll need to ignore more than a decade of bad memories to finish the job though, a fresh-faced McIlroy four shots clear in the final Masters round in 2011 before shooting an 80.
His final-round partner will cut an imposing figure too, the popular DeChambeau boasting bragging rights after McIlroy's most-recent major heartbreak at Pinehurst last year.
The American held his nerve to snatch the US Open after McIlroy bogeyed three of his last four holes to give up a two-shot advantage.
Australia's other cut survivor Min Woo Lee (77) had a day to forget, penalised a stroke for accidentally contacting his ball on the 13th and earlier copping a warning for slow play to finish four over.
Defending champion and world No.1 Scottie Scheffler carded an even-par 72 to sit level with Day at five under.
16°C