Phil Nunnerley, 69, was on a bus from the England v Fiji match at Twickenham when he collapsed last Friday.
He has launched an appeal in a bid to track down his "saviour", who performed CPR for 15 minutes.
Mr Nunnerley, from North Somerset, said the only clue to the rescuer's identity was his Gloucester Rugby Club shirt.
"I remember nothing of it, from the moment I got on the bus until I woke up in the ward. I was oblivious to it all," said Mr Nunnerley.
"I had a man there who worked hard for 15 minutes with CPR and I owe my life to him and would dearly like to thank him."
Mr Nunnerley's partner, Rosemary Henderson, who accompanied him to the match, put out the appeal on Facebook.
The couple, who are from West Harptree in the Chew Valley, were sitting apart as it was a busy bus.
But she noticed he "didn't look quite right, he seemed to be nodding off and going to sleep".
She shook him but he did not respond, causing her to panic and call out for help.
"A man came forward and said I'm a doctor," she said.
"He obviously recognised that Phil had had a heart attack, and immediately said get him onto the floor."
They were "extremely lucky", she said, as there was a nearby ambulance crew who met them when they arrived at the hospital car park to take over.
She now wanted to learn CPR as a result of her partner's ordeal, she added.

The defending champions were all out for 282 on day one, but were 51-6 at one stage during the morning session.
Hodd put on 132 with Azeem Rafiq (74) and 88 with Jack Brooks (48) but was left on 96 not out when Brett Hutton (3-67) had Ryan Sidebottom lbw.
Notts lost Jake Libby and debutant Tom Moores as they closed on 38-2 in reply.
Yorkshire went into the game second in the table, but needing a victory to close the 26-point gap between themselves and leaders Middlesex, who are not playing this week.
Steven Mullaney (3-57) ran out Adam Lyth backing up in the ninth over and then proceeded to remove Alex Lees, Jake Lehmann and Tim Bresnan with his medium-pace.
Hodd and Rafiq, who was capped during the day, led the fightback over the next 33 overs before the latter was lbw to Samit Patel.
Brooks, however, played his part in their revival with his highest score of the summer before sending back Libby lbw with his opening delivery of Notts' reply, and Bresnan had Moores caught at slip with the final ball of the day.

It was not immediately clear what had happened, with spokeswoman Heather Nauert saying there were no "definitive answers about the source or cause".
But Ms Nauert refused to be drawn further on what the symptoms were.
But one source told the Associated Press news agency the symptoms reported included hearing loss.
It was suggested the hearing loss could have been caused by some kind of sonic device placed outside the diplomats' homes, emitting inaudible sound waves that can cause deafness.
Staff began complaining of the strange symptoms late last year, Ms Nauert said.
While they were not life-threatening, she revealed a number of people had been brought home to the US as a result.
Ms Nauert said the government was taking it "very seriously, and there is an investigation currently under way".
It is understood that the two expelled Cuban diplomats left Washington DC in May, the BBC's Will Grant reports from Havana.
So far, their government has not responded to the reports.
Washington and Havana only re-established ties in 2015, following 50 years of hostilities between the two countries.
A rugby fan who suffered a heart attack while returning from the World Cup opener is seeking a mystery doctor who helped to save his life.
[NEXT_CONCEPT]
Andrew Hodd ran out of partners just short of a century after rescuing Yorkshire from a dismal start to their Championship game at Scarborough.
[NEXT_CONCEPT]
Washington has expelled two Cuban diplomats after US embassy staff in Havana suffered mysterious physical symptoms, the US state department said.