Heroic rescue ... Alton Brieske jumped into a Florida lake to rescue an elderly driver trapped in his car. Picture: WPBF Source: Supplied

Alton Brieske, 21, was having lunch when he saw a car veer off the road and plunge into a lake in Port Lucie, Florida, on Wednesday.

Without a second thought, the Florida Atlantic University premed student took his shirt off and jumped in.

Water poured into the vehicle as the driver, Salvatore Mancuso, sat unconscious behind the wheel.

“When that happened, that’s when I was like, ‘All right, we need to get him out of here, because it’s going to sink any time now’,” he told US TV station WPBF. “I started screaming, ‘I need a hammer! I need a hammer!’ Then some guy comes running through the bushes, and he’s like, ‘I got one’.”

Video shows Mr Brieske smashing the window with the hammer and dragging Mr Mancusco to shore.

“I broke that window, popped the lock, opened the door,” he said. “That’s when the water started rushing in, so I just acted fast, popped his seatbelt off, put my arm under his shoulder and got him out of the car.”

Mr Brieske performed CPR on Mr Mancusco, reviving him before he was rushed to hospital in a critical condition.

CPR ... Alton Brieske performed CPR on the unconscious driver, reviving him on the banks of the lake. Picture: WPBF Source: Supplied

Mr Brieske, who dreams of becoming an ER doctor, refuses to be called a hero.

“People are saying, ‘You’re a hero.’ I’m not a hero,” he said. “It’s a heroic act, I guess you could say, but I was just in the right place at the right time. That was my calling for today was save this guy.”