Petrov in positive mood ahead of treatment
Hartson, who beat testicular cancer in 2009, said he believed the 32-year-old Bulgaria midfielder's fitness would help him in his fight against the disease.
"He's the kind of lad who wants to tackle this head on," the former Wales striker told the BBC.
"He's very strong. His fitness, I'm sure, will help him. He knows he has so much support throughout football."
Hartson said he spoke to Petrov on Sunday before starting his treatment at a London hospital.
"A lot of it is psychological," added the former Arsenal forward. "You have to get your head around what is happening and deal with it as openly as you can.
"You know there is a lot of support out there when you are battling a near-death experience like I went through.
"Stiliyan knows he has support from his peers and his family which will be very important for him," added Hartson.