Resuming the day on 62 for no loss after bundling India out for 148 yesterday, England scored at a brisk pace by adding 323 runs today from 86 overs to end the day on 385 for seven at an Oval pitch which has eased out considerably.
Root was going strong on 92 not out after facing 129 balls with the help of nine fours and a six. Chris Jordan was giving him company on 19 and the duo have put on 67 runs for the unbroken eighth wicket.
Captain Alastair Cook and Gary Ballance also made substantial contributions with 79 (off 183 balls) and 64 (off 117 balls) respectively.
'Butter fingered' Indian fielders also did not do any good for their team's cause by dropping Cook twice -- on 65 and 70.
India, who are trailing the five-match series 1-2, will now need a herculean task to save the Test.
Indian bowlers gave a sort of a fightback in the second session as they took four wickets through a double strike by off-spinner Ravichandran Ashwin and a wicket each by pacers Varun Aaron and Ishant Sharma.
But they were not able to find regular breakthroughs after tea break and let go off the opportunity to claw back into the game.
Ashwin (2/55), Aaron (2/111) and Ishant (2/58) took two wickets apiece, while Bhuvneshwar Kumar (1/86) also chipped in on the day. Stuart Binny (0/58) was the other bowler deployed.
Root and Jos Buttler (45 runs, 73 balls, 9 fours) put on 80 runs for the sixth wicket after tea as they wrestled back the initiative from Indian bowlers.
The Indian attack lacked consistency in the final session in comparison to their bowling in post-lunch session and the two batsmen took advantage of that to score freely. Root and Buttler brought up their 50-run partnership in the 82nd over, the runs coming off from only 81 balls.
The 300-run mark came up in the 91st over and England were looking comfortable again at that point. Buttler was then dismissed by Ishant, caught at short mid-wicket, against the run of play. It became two quick wickets as Chris Woakes (0) as Kumar used the second new ball to open his wickets' column this innings.
But then Root and Jordan got stuck in, not allowing any chances to come through. Root reached his fifty off 93 balls, with two fours and a six, and then opened up to play some strokes taking 59 runs off the last 34 balls of his innings.
At the same time, he made sure that there were no other hiccups for his side as he and Jordan saw off the rest of the session.
Earlier, Ashwin took two quick wickets as India pegged back England late in the post-lunch session. This was after Cook's half-century had helped level the scores. The skipper continued his 82-run partnership with Gary Ballance and they further added 43 more runs, totalling a second-wicket stand of 125 runs.
Three overs into the session -- 48th of the innings -- Ballance reached his fifty off 77 balls, including 11 fours and an over later celebrated his 100-run partnership with Cook. Then began an odd period of play wherein India could have had more wickets but for the two catches dropped.
The first was in the 50th over when Cook was dropped off Aaron by Murali Vijay at first slip. The English captain was batting on 65 then. He had another reprieve five overs later when Ajinkya Rahane dropped him while on 70 off Ashwin this time, again at first slip.
Finally some luck came the Indian bowlers' way as Vijay held on to another chance offered by Cook in the 58th over bowled by Aaron. He was finally dismissed for 79 runs off 183 balls with nine fours included. England's 200-run mark came up in the 62nd over thereafter but more importantly they collapsed from 191/1 to 204/4 in the space of 33 balls.
Ballance was the first to go, caught by Cheteshwar Pujara at silly point off Ashwin, the off-spinner finally getting his first overseas Test wicket since 2011-12. Three balls later, Ian Bell (7) received a great delivery from Ishant and was duly pouched behind by the wicket-keeper.