KIEV, Ukraine -- Ukraine's presidential candidates on Friday took alcohol and drug tests, part of an increasingly fierce campaign fight before a runoff vote.

Volodymyr Zelenskiy, a popular comic actor who left President Petro Poroshenko far behind in Sunday's first round of voting, had challenged the incumbent to take the test.

Poroshenko took the test Friday at Kyiv Olimpiyskiy Stadium, the country's biggest arena, where the two rivals are set to debate two days before Ukraine's April 21 presidential runoff.

Poroshenko challenged Zelenskiy to take the test at the sports arena along with him, but Zelenskiy said he doesn't trust that lab and took his test at another one.

"I took the blood test," he told journalists later. "I have enough blood, fresh blood."

The head doctor at a city narcological clinic, Volodymyr Yariy, said initial analysis of Poroshenko's tests showed no presence of alcohol, Ukrainian news reports said. Zelenskiy issued a report from his clinic showing no presence of psychoactive drugs, according to the news agency UNIAN.

Zelenskiy, who stars in a TV series about a teacher who becomes president after a video of him denouncing corruption goes viral, has made the fight against corruption in Ukraine the focus of his campaign. He won 30% of Sunday's vote, while Poroshenko got 16%.

Poroshenko accepted Zelenskiy's challenge to have their debate at the sports arena instead of a TV studio.

Poroshenko, 53, has argued that Zelenskiy's lack of political experience makes him unfit to stand up to the pressure from Russia, which annexed Ukraine's Crimean Peninsula in 2014 in a move that most nations don't recognize and cast its support behind separatist rebels in eastern Ukraine.

Zelenskiy, 41, has rejected the contention, accusing Poroshenko of bringing the country to ruin by failing to end the separatist conflict in the east, or shore up the sagging economy or stem rampant corruption. His lack of political experience helped his popularity with voters amid broad disillusionment with Ukraine's political elite.