Owen Batty, who is based at the Newport East Delivery Office and is also a crew manager with South Wales Fire and Rescue Service, was responding to an emergency with his crew when they spotted a collapsed driver in a car on the Wye Bridge in Monmouth.
Mr Batty recognised the man as the father of a close friend.
The man, 58-year-old Julian Vaughan from Monmouth, was in cardiac arrest.
Mr Batty and his team immediately began CPR and used a defibrillator at the scene.
Mr Batty said: “When we attached the defibrillator, Julian received two shocks.
“After the second shock, he began to cough and regain consciousness. By the time the ambulance arrived, he was talking.”
A paramedic later confirmed that without the immediate CPR and defibrillation, Julian would not be alive today.
Mr Vaughan said: “I honestly believe I wouldn’t be here without the quick actions of everyone on the scene.
“The CPR saved my life.”
He has since had two stents and a balloon fitted.
Mr Vaughan, who previously trained as a workplace first aider, now encourages others to learn CPR.
Meanwhile, the incident has strengthened Mr Batty’s dedication to teaching lifesaving skills.
He has trained more than 3,000 people in CPR and supports Royal Mail’s partnership with the British Heart Foundation (BHF).
He said: “RevivR is a brilliant way to help people understand what cardiac arrest is and why immediate action matters.
“It gives people the confidence to step in, even in frightening situations.”
He is urging the public to take just 15 minutes to learn CPR through RevivR.
“A few minutes of learning can make all the difference,” he said.