England 477 and 12 for 0 trail India 759 for 7 dec (Nair 303*, Rahul 199) by 270 runs
Live scorecard and ball-by-ball details
Back in March 2008, the MA Chidambaram Stadium witnessed the first triple-hundred on Indian soil, as Virender Sehwag plundered 319 against South Africa. Eight-and-a-half years later, the stadium's revamped stands became the backdrop to the first triple-hundred by any Indian batsman apart from Sehwag. That batsman, Karun Nair, was playing his third Test match, and was only playing because India's middle order had lost two of its regular occupants to injury.
When India next play a Test match, they will need to choose who to leave out - and perhaps even which two - among Ajinkya Rahane, Rohit Sharma and Nair, a man with an unbeaten 303 in his last innings. Three hundred and three, not out. A square-cut brought up the landmark; Alastair Cook had brought all his men into single-saving positions with Nair on 299. Adil Rashid dropped short, Nair slapped it away, and Cook just happened to be the fielder diving uselessly to his left from cover point.
The declaration came right then, with India 759 for 7. It was their highest-ever total, against anyone. It was the highest total against England, by anyone. It left England, starting their second innings with a deficit of 282, 16 minutes to get through to stumps.
By the time Virat Kohli called his batsmen off the field, they had inflicted as much mental disintegration upon England as they have faced anywhere in the time since Carl Rackemann coined the term during the 1989 Ashes. At lunch, India still trailed by 14 runs. At tea, they led by 105. So far, so Mumbai, on a pitch that was rather flatter than Mumbai, and England didn't seem in any immediate danger of defeat. By the time India declared, an innings defeat wasn't out of the question. Alastair Cook and Keaton Jennings got through to stumps unscathed, but their task has barely begun.
England have it all to do on the last day of a sapping tour of the subcontinent. This is still a flat pitch, by the standard of Indian pitches, but R Ashwin and Ravindra Jadeja bowling on it with raucous voices clustered around the bat is an entirely different prospect to England's spinners bowling with five fielders on the rope.
Ashwin and Jadeja did their bit with the bat as well, scoring 67 and 51 as the sixth- and seventh-wicket partnerships added 319 to India's total. Post-tea, India clattered 177 runs in 25.4 overs. That's 6.9 runs per over. Nair, on 195 at tea, scored his last 108 runs in 78 balls. If his triple-hundred wasn't uniformly Sehwagian in tone, it certainly was now.
England tried to bounce him with a fine third-man - almost a long stop - in place for the ramp over the keeper. Nair tennis-forehanded Jake Ball through the vacant mid-on region. Then he played the ramp anyway, against Stuart Broad, and it carried all the way for six. When the spinners returned he reverse-swept Moeen Ali, and tonked four fours and a brutally clubbed six off successive overs from Adil Rashid. This when Rashid had five men on the boundary.
Nair's innings wasn't chanceless, of course. Cook had put him down at slip on day three, a hard chance flying to his right when Nair edged Ball on 34. Then, on 154, he had tried to reverse-sweep Rashid and sent the ball into Jonny Bairstow's gloves. Replays and Ultra Edge suggested it had deflected off the face of his bat. The umpire said not out, and England had no reviews left.
On 217, he edged Ball again, the third new ball going low to Joe Root's right at slip. He only got his fingertips to the ball. And finally, on 246, he stepped out, heaved at Moeen, missed, and turned around to see the unsighted Bairstow fluff the stumping chance. Destiny was clearly on Nair's side, and he chased it in a hurry, scoring 57 off 39 balls after the missed stumping.
All this merrymaking, of course, would not have been possible without the restraint he displayed on day three - he walked in with India 211 for 3 and still trailing by 266 - and in the morning session of day four. India went at less than three runs an over in the session before lunch, but lost only one wicket, M Vijay lbw to a Liam Dawson arm ball.
Resuming on 71, Nair took 49 balls to reach his maiden Test hundred. M Vijay saw him through a nervy period in the nineties, exhorting him from the other end to stay calm and wait for the scoring opportunity. Having played out five dots from Ben Stokes on 99, he reached the landmark by defying a packed off-side field, which included two short covers for the uppish drive, stretching out to a full, wide ball and letting it come to him to steer it past the diving backward point fielder.
That was only the third boundary Nair hit in those first 49 balls - and one of them had been unintentional, off an edge when he tried to leave the ball. It reflected the hard-nosed approach India had had to take in a session where England set defensive fields, bowled with discipline, and got a bit of help from the surface, largely through inconsistent bounce. Dawson nearly bowled Vijay with one that crept low, and Ben Stokes, hitting the pitch hard, got the ball to lift disconcertingly as lunch approached, taking a chunk off the shoulder of Nair's bat and hitting Ashwin's right glove.
Cook's use of his spinners also contributed to India's caution in the first session: he bowled the accurate Dawson unchanged from one end - he sent down 13 overs for 31 runs - and his seamers from the other, only using Rashid for one over - the last one before lunch - and not using Moeen at all.
Rashid's introduction brought a little spike of aggression from Nair, who made himself a bit of room and drove him inside-out to the cover boundary. It was just a teaser of what was to come after lunch.
Karthik Krishnaswamy is a senior sub-editor at ESPNcricinfo