Ukraine’s president, Petro Poroshenko, has agreed to give blood for a drug test before a debate at the capital’s Olympic stadium with his rival in the country’s made-for-TV elections.

It was not immediately clear whether Volodymyr Zelenskiy, a comic who plays Ukraine’s president on his own television show, had expected Poroshenko to accept his challenge when he proposed the debate.

But it now may be too late for him to renege on the deal.

Zelenskiy is the frontrunner in the 21 April runoff, having won Sunday’s first-round elections with 30% of the vote – nearly double the amount received by Poroshenko.

The comic, when pressed by journalists during a champagne-soaked celebration in downtown Kyiv, said he was ready to debate the president. Critics have said Zelenskiy is a political neophyte and is unprepared to run the country. He has given few public statements and just a handful of interviews.

On Wednesday, his team released a slickly produced video showing the candidate walking into the centre of a stadium while issuing a direct challenge to Poroshenko. “You invited me to a debate,” Zelenskiy said. “You hoped I would run away … no. I am not you four years ago.”

Volodymyr Zelenskiy at his campaign HQ after Ukraine’s first-round presidential election put him first place with 30% of the vote. Photograph: Valentyn Ogirenko/Reuters

In 2014, Poroshenko declined to debate his then-opponent, Yulia Tymoshenko.

Zelenskiy’s conditions were simple, if unusual: the debates would be held at the Olympic stadium, broadcast live on all television stations and all candidates would have to undergo medical testing to show they were not “an alcoholic or drug addict”.

It was not immediately clear if it was a serious invitation. The stadium’s press service said in a statement it had not been approached by anyone before Zelenskiy issued his challenge. And most expected Poroshenko to shrug off the challenge.

But on Thursday morning, Poroshenko shot back. “This is no joke,” he said in his own video, adding the elections were “not a competition to be liked”. He said he was ready for the debate: “But if you want a stadium, let there be a stadium. I am waiting for you.”

Poroshenko’s press team announced that he would have his blood tested at 9am on Friday at the Olympic stadium and official accreditation has been opened for journalists. But late on Thursday afternoon, Zelenskiy’s team announced that they were not satisfied with the Ukrainian president’s choice of clinic, and demanded he go to another one.

Adding to speculation that the challenge may have originally been a joke, Zelenskiy’s camp released a second video on Thursday evening, suggesting the debate be moderated by Tymoshenko, who came in third in the first round.