The 29-year-old smashed an unbeaten 309 in the first Test against South Africa to spur India to 468 for one, just 72 runs away from the visitors' total.
Sehwag's 278-ball 300 was the fastest since number of balls faced began to be recorded and matched the feats of Australian Don Bradman and West Indies' Brian Lara, the other batsmen to score Test triple hundreds twice.
The only Indian to score a Test 300 matched his score against Pakistan at Multan in 2004, now eyeing Lara's record 400 not out.
"I have no words to describe this knock," Sehwag said after the third day's play. "This innings is even better than the one in Multan. "I had to prove myself that I belong at this level."
Sehwag hit 42 fours and five sixes, batting for more than eight hours in sapping heat and humidity, needing ice baths at the lunch interval and after stumps.
The batsman was dropped because of a form slump last year and made the squads for the 2007 one-day World Cup and the Australia Test series only because captains Rahul Dravid and successor Anil Kumble insisted on his selection.
He vindicated Kumble's faith after being recalled for the last two Australia tests, scoring 151 in the drawn final game in Adelaide in January.
"When I was dropped I was hurt because I had a very good Test record," said Sehwag, who averages more than 50. "I wanted to prove that I am a better Test player."
Sehwag, who notched his 14th triple-figure knock, rated the effort ahead of Multan because of the oppressive weather.
"It is very difficult in Chennai," he said. "Because of the humidity you are sweating a lot and losing lot of body salts."
Sehwag said he would have a good night's sleep before thinking about breaking Lara's record.
"Tomorrow, I will try and bat one session," he said. "If I bat till lunch I will come close to Lara's record."
He felt if India took a 200-run lead, they could push for a win.
South Africa coach Mickey Arthur was deflated after India scored 386 runs in a day.
"We were beaten by one chap today," he said. "In my wildest of dreams, I didn't think they would score this quickly. "It was all because of one man. I tried to visualise every scenario last night but this was not one of them.
"We tried to do everything -- bowled wide, over the wicket, round the wicket, nothing worked."