Richard Loach performed CPR on Phil Nunnerley, 69, who had a suspected heart attack on a bus after the match against Fiji at Twickenham last Friday.
Mr Nunnerley launched a social media campaign to try to track his saviour down. He only knew he was wearing a Gloucester Rugby shirt.
The men have now spoken.
Dr Loach, a GP on the Isle of Wight who used to live in Gloucester, got in touch with Mr Nunnerley after reading about his story in a BBC News website article.
He said: "Phil's a very lucky man. It was so great to see him looking so well on that short video [in the BBC story].
"I've spoken to him and he sounds so great. My wife and I were both involved and we were so chuffed to see him so well."
Dr Loach said he and his wife realised Mr Nunnerley was ill while they were travelling on the bus.
"We were just minding our own business and we heard someone say 'stop the bus'.
"He didn't look very well. I had seen it before. We thought we'd better help out."
Dr Loach said Mr Nunnerley, from West Harptree in North Somerset, had no pulse when they started carrying out mouth-to-mouth resuscitation and chest depressions.
Four paramedics then continued to treat him when the bus got to the park and ride.
"Normally the outcome is not usually so good. I'm sure anyone else would have done exactly the same," said Dr Loach.
Mr Nunnerley said he is planning to meet soon to say thank you in person.
A doctor who gave emergency first aid to a rugby fan after England's opening World Cup match has said the man he treated is "very lucky" to be alive.