Sehwag, who scored 309 in the first Test against Pakistan, told IANS in an interview.
"He said such an opportunity comes once in a player's career, 'and you are lucky that it has already come in yours'," recalled the 25-year-old, who, like almost everyone, holds Tendulkar in high esteem.
"Sachin also said that there are so many players who have retired without getting a triple century," said Sehwag, speaking just before going abroad with wife Aarti for their honeymoon.
Added the New Delhi-based player: "Sachin made me keep my cool as there was a lot of time at hand. It is only then I realised that 'Yes, such opportunities don't come often in a career'."
With a 1,200-gram bat called 'Ton', Sehwag batted for eight hours and 51 minutes and smashed 39 boundaries and six sixes at the Multan Cricket Stadium to make history.
Sehwag, often compared to the maestro for resemblance in their strokes, said even if Tendulkar had not been at the crease, he would have sent a message to him during drinks.
"If he had not been there, maybe I might not have scored a triple century... I would have got out trying to smash fours and sixes. But perhaps it was destined that Sehwag would score a triple century and that Sachin would be around when I did that," said the right-hander who reached 300 with a six off Saqlain Mushtaq.
"I was so happy, also because that my role model Sachin Tendulkar was batting with me. He congratulated me, hugged me -- and perhaps he was more happy than me," said the affable Sehwag, who broke V.V.S. Laxman's Indian record of 281.
Sehwag, who opens the innings with Tendulkar in one-dayers, added: "The right person was there at the right time, and he gave me the right advice."
He said he realised the significance of his achievement after the match ended in an innings and 52-run win for India.
"It was when everyone started congratulating me and I started getting letters and phone calls," he recalled.
"Letters and faxes came from (Indian cricket chief) Jagmohan Dalmiya, Sahara India, Hero Honda, Coca Cola. Pawan Munjal (Hero Honda chief) sent me a bouquet and two bottles of champagne. Lots of relatives and friends also called up."
Sehwag was not an outstanding success in the one-day series, which India won 3-2, but in three Tests, he scored 438 runs at 109.50 -- the top aggregate on either side.
Sehwag said he had not set any goals himself for the Pakistan tour.
"I went there with a blank mind. I only thought of playing well and performing," he averred.
With a majestic 195, scored in the Melbourne Test against Australia in December, under his belt, Sehwag went to Pakistan riding high on confidence.
"I was thinking that when I have performed well on the hard and bouncy tracks of Australia, maybe it would be slightly easy in Pakistan," he said.
"The thought of playing Pakistani bowlers in Pakistan ... reverse swing etc ... there was a little bit of apprehension initially. But I was confident of my ability and just carried on with the same form," he said.
There were other fears as well on the historic 39-day tour - those of security, on and off the field, and on how people would behave with the players.
"But after our arrival in Pakistan, and after we played the first one-day practice match at Lahore and started travelling, we met a lot of good people, and got a lot of affection and support from them."
"When you are on a tour all the confidence that comes is from the practice games. If you score runs in practice games, you think, 'yes I am in touch, in form and I will perform well in the matches ahead,' and want to continue with the same form."
Before the tour, Sehwag had spent a couple of days at nets under the eyes of his childhood coach Amar Nath Sharma at his alma mater, a government school in New Delhi's Vikaspuri neighbourhood.
"I went to him so that he could point out the changes, the mistakes that might have crept into my batting," he said. "He would know about these things more than anyone else because he has seen me bat since my childhood."
Sharma advised Sehwag to occupy the crease if he wanted to score runs in Pakistan.
"He said you will have to play with patience and if you do that you could score runs," he said.
Sehwag eventually showed a mature head and it paid rich dividends.