The 41-year-old actor and comedian Zelensky is favourite to become Ukraine's next president after he dominated a first-round vote on March 31 despite many initially dismissing his candidacy as a joke.
The two will meet in a run-off vote on April 21.
Hours after political novice Zelensky challenged Poroshenko, 53, to hold a debate in a Kiev stadium, the country's largest sports arena, Poroshenko said he was ready to square off with the popular actor.
"(If you want) a stadium, let it be a stadium," Poroshenko said in a video address.
But he also warned that an election debate was no laughing matter and a presidential election would determine the country's fate for decades to come.
"A debate is not a show," Poroshenko said in an apparent jab at Zelensky's lack of political experience.
"A debate is not staged for a stadium spectacle," he added.
Zelensky's political experience had been limited to playing the president in a TV show when he announced his long-shot candidacy at the start of the year.
But he capitalised on frustration over corruption and a stalling economy to leapfrog his establishment opponents.
Addressing Poroshenko, Zelensky listed a number of conditions, saying he should stop calling him a "clown" and a "puppet," a reference to the frontrunner's alleged political ties to oligarch Igor Kolomoisky.
By law, televised election debates are to be held on the last Friday before the second-round runoff, which would be April 19.
With 100 percent of the ballots counted, Zelensky is ahead with more than 30 percent of the first-round vote, nearly twice that of Poroshenko, on almost 16 percent.