The 25-year-old, a former top-ranked amateur, has two European Ryder Cup stars as his nearest challengers with Tommy Fleetwood (69) in second place and Shane Lowry (69) a stroke back in third after Saturday's third round.
No.2-ranked Rory McIlroy, the main attraction at Delhi Golf Club, shot 68 and was in a tie for 15th place — seven strokes off the lead heading into the final round.
"I'm probably two shots too far behind to have a realistic chance," McIlroy said.
Nakajima birdied four of his first eight holes, and then three in a row from the 12th over a combined 70 feet to jump clear of then-leader Fleetwood.
He parred his way home to retain the lead in his bid for a first title on the tour since March 2024, which also came in India at the Hero Indian Open.
Nakajima said the secret to his good third round was being able to watch his playing partner, Lowry.
"I love his swing tempo," Nakajima said, "and I copied him. So that's why my swing (was) better than yesterday. And mindset was great."
Fleetwood, who had held the overnight lead, birdied two of his first three holes and also the par-5 eighth. His only bogey came at the 17th after three-putting from 30 feet, but he made amends with a birdie at the last to make it into the final group.
"Keita is not far enough ahead where he's completely in control," said Fleetwood, who won the Tour Championship on the PGA Tour in August and was the top points scorer for Europe in its Ryder Cup victory last month.
"Just go into tomorrow, enjoy it. Final group is always great. You've got to enjoy those moments. You don't know when they're going to come again," he added.
Australian Jason Scrivener followed his second-round 69 with a 71, to leave his as one of five players on three-under.
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