The 30-year-old got the better of his Argentine opponent 6-3 3-6 6-4 at the O2 Arena in London to begin his Group B campaign on a winning note.
"I am very, very, very happy," the Spaniard is quoted as saying by the BBC. "It was a top match, and it was very difficult to play against Juan Martin. I hope the people in here enjoyed our tennis.
"I don't know the secret [of my good form]. It has been the best season of my career. Maybe it's because I didn't have a big injury."
Ferrer will take on Roger Federer on Thursday.