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.
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.