KIEV, Ukraine — The Ukrainian presidential race took a surreal turn on Sunday when what was supposed to be a debate between the two candidates featured President Petro O. Poroshenko discussing Ukraine’s future with an empty lectern bearing the name of his rival, a comedian.

His rival, Volodymyr Zelensky, had made clear beforehand that he would not attend the debate because of a quarrel over timing. But the president, struggling to revive his flagging re-election campaign, went ahead anyway, assailing his opponent as an unserious “virtual candidate.”

The one-man “debate,” held in a stadium complex in Kiev, the capital, and broadcast live on television, came just a week before the second and decisive round of an election that has been dominated from the start by theatrical stunts and dirty tricks rather than policy issues like how to settle a five-year-old war in the east of the country.