"I didn't know if I was still going to be playing. I predicted that I wasn't going to be playing anymore at 38, 39, and here I am," Federer said at a joint press conference with Zverev at a hotel in Buenos Aires's Puerto Madero neighbourhood.
"I'm really actually happy that I feel the way I am. I've had two great years physically also in the last two years."
Federer has long been part of a remarkable triumvirate along with Spain's Rafael Nadal and Serbia's Novak Djokovic that have pushed one another to greater heights while winning a combined total of 55 Grand Slam titles and keeping Zverev and other talented young players titleless at tennis' four most prestigious events, reports Efe news.
"Every generation brings something new to the table, maybe in power, maybe in movement, maybe in tactical elements, and I'm still feeling I can ... always improve. You can always learn and that's what I'm doing," the Swiss world No. 3 said.
Federer, who has won a record 20 men's singles Grand Slam titles, but now is only one ahead of the 33-year-old Nadal, said he also will use Wednesday's exhibition match at Buenos Aires's Parque Roca Stadium as an opportunity to experiment with his game.
"I'm not just going to go out there and play a little bit of tennis. I'm actually going to go in there, enjoy myself, but also maybe try out a few things."
Referring to his eventual retirement from the sport, Federer said he will play it by year.
"There's no rules to it. You go by feel, and I actually don't know if I'm going to announce it early, late, all of a sudden. I think it all depends on my health, on my family, on my results also a little bit, obviously."
The 22-year-old Zverev, for his part, said the exhibition match will be an emotional one for him, Federer and the Argentine fans.
"All of us players, you know, most of the time we lose to him, but we still hope he doesn't retire. We still want him around as much as possible," the world No. 7 said.
Federer was originally slated to play Juan Martin del Potro, but the popular Argentine had to pull out of the exhibition event due an injury setback.
Before the press conference got under way, Federer greeted nearly 100 fans who had gathered outside the hotel and signed autographs and posed for photos.
Federer and Zverev are playing the exhibition match instead of competing in this week's revamped Davis Cup Finals.
Switzerland did not qualify for that event (being played in Madrid) mainly due to Federer's lack of participation in the qualifying phase in February, while both he and Zverev have been critical of the drastic overhaul of that 119-year-old competition.
( With inputs from IANS )