England were 251-4 at stumps, with Root coming in when they were faltering at 44-2 after paceman Nitish Kumar Reddy struck twice in his first over to remove openers Ben Duckett and Zak Crawley.
Not for the first time in his 156-Test career, the 34-year-old Root dug England out of a hole, at a run-rate of just above three an over, well below the current 'Bazball' side's usual standards.
England captain Ben Stokes, who won the toss at a sun-drenched Lord's, was 39 not out after helping Root, his predecessor as skipper, add an unbroken 79 for the fifth wicket.
Root also shared a grinding partnership of 109 in 35 overs with Ollie Pope, who made 44 after almost being out for a golden duck.
"It's not always been the way we've gone about our cricket, but I think long term in the game hopefully that will reward us if we can kick on tomorrow," Pope told Sky Sports.
Much of the pre-match talk had focused on the return of Jasprit Bumrah, the world's top-ranked Test bowler, after he was rested for India's 336-run thrashing of England at Edgbaston last week that levelled the five-match series at 1-1.
But it was Reddy, the fourth seamer deployed by India captain Shubman Gill on Thursday, who struck twice in four balls on the way to figures of 2-46 in 14 overs.
With his third ball, the 22-year-old had left-hander Duckett (23) well caught down the legside by diving wicketkeeper Rishabh Pant off a gloved pull.
Next ball, the often skittish Pope almost fell for a duck when he threw his bat at an outswinger, with Gill just unable to hold a tough chance low to his right in the gully.
But with the last ball of the over, Reddy produced a superb full-length delivery that swung late and straightened off the pitch before taking Crawley's outside edge to give Pant a simple catch as the Kent batsman fell for 18.
Root masterclass saves the day for England
Pope, following his first-ball reprieve, had several more nervy moments, chasing a wide delivery from Bumrah, he could have left alone, before a loose shot off Reddy fell just short of third slip.
Root, second behind England teammate Harry Brook in the Test batting rankings, was far more assured as he went to fifty with a glance through fine leg off Reddy - his seventh four in 102 balls faced.
England were 153-2 at tea with Root unbeaten on 54 and Pope 44 not out.
But the very first ball after the interval saw Pope drive loosely at left-arm spinner Ravindra Jadeja, with reserve wicketkeeper Dhruv Jurel, on the field after Pant suffered a finger injury in the second session, holding an excellent shoulder-high catch off a fast-travelling edge.
New batsman Brook only managed 11 before he was bowled between bat and pad by Bumrah after failing to get forward to a nip-back ball. Bumrah ended the day with a miserly return of 1-35 in 18 overs.
England were now 172-4 as Stokes, without a Test hundred since an Ashes century against Australia at Lord's two years ago, walked out to bat.
The left-hander had made 27 when Reddy had a tight lbw review turned down on umpire's call. He then called for the England physio after suffering what appeared to be a groin injury.
Root, however, continued to bat with his customary class, an on-driven four off Reddy, taking the former England captain to 80.
With play held up when ladybirds descended upon the ground, and Stokes unable to get off strike as he defended Akash Deep's final two balls of the day, Root was left one run shy of what would be his 37th Test hundred and a record-extending eighth at Lord's.