Stephen Martindale, 40, a former Royal Marine from Horwich, was cycling home with a friend after visiting Southport on Friday, April 10.
As they reached Bretherton, a van which was travelling in the opposite direction veered off the road and into a ditch.
The driver, who wishes to remain anonymous, crashed after losing-consciousness at the wheel at around 6.30pm.
At first, Stephen and his friend thought the noise of the crash was a tractor exiting a field, but as they turned the corner, they quickly realised the sound was coming from the van's engine.
“The engine was screaming," Stephen said. "My friend stayed on the road to look after the bikes as he's not from the military or anything, so I just jumped in."
After checking the driver’s pulse, breathing and heart rate, Stephen pulled them out of the van and started CPR.
Stephen said: "I'd be working in the Middle East for 13 years up until a couple of years ago.
"I had done a lot of medical training while I was there, and in the Marines, so it just kicked in.
"The passenger was on the phone to 999 so I dragged the driver from the van and started compressions as he had no pulse."
If you come across someone who is unconscious, unresponsive, not breathing or not breathing normally, they’re in cardiac arrest.
The most important thing is to call 999 and start CPR to keep the blood flowing to the brain and around the body, according to the British Heart Foundation.
After a cardiac arrest, every minute without CPR and defibrillation reduces someone's chance of survival by 10 per cent.
“It was about 20 minutes before paramedics arrived," Stephen explained. "There was a nurse and her husband who were driving past as well and she jumped in too.
“She did about two or three minutes of chest compressions and I jumped back on while she was on the phone.”
While administering CPR, Stephen also made sure the passenger of the van was safe and kept them occupied to stop them from going into shock.
When ambulance crews arrived at the scene, paramedics used a defibrillator to shock the driver's heart back into a normal rhythm.
“They loaded them onto a stretcher and then onto a trolley as we had to carry them out over the bushes and the trees," Stephen said.
"Then they loaded them onto an ambulance, the police took my details and then I had to cycle back home for an hour."
After arriving home, Stephen tried to contact different hospitals to see if he could get an update about the driver's condition, but he only had their first name which meant he was unable to get any more information.
On Monday, April 13, three days after the incident, one of the driver's sons launched an appeal on social media to try and find Stephen to thank him for saving his father's life.
"A friend of mine told me about the post," Stephen said. "They rang me up to ask if it was me as I’m the only person they know who is an ex-Royal Marine with an Iron Man tattoo.
"He posted my phone number on the post and told me someone might be in touch, and that's how they found me.
"I'm glad their family contacted me because it did worry me. 'Have they survived?' is one of those questions you'd like to know the answer to."
They now plan to meet up after the social-distancing rules are lifted.
Stephen said: "They said they have a massive family of sisters and brothers, nieces and nephews, sons and daughters who want to say thank you, so when this is over, we're going to meet each other.
“It was like a series of events that never usually happen to get me on that road, but if we hadn't of gone past, they probably wouldn't be with us today."