Many Ukrainians were up in the early hours of Sunday morning, for once not to seek shelter from incoming Russian missiles, but to celebrate the Ukrainian boxer Oleksandr Usyk becoming the world’s undisputed heavyweight champion.

Mr. Usyk’s victory over the British boxer Tyson Fury was a rare piece of good news for an embattled nation that is struggling to contain Russian advances, particularly in the northeast, where Moscow has opened a new front.

President Volodymyr Zelensky lauded the victory as a symbol of Ukraine’s resilience.

“Ukrainians hit hard!” Mr. Zelensky wrote in a Telegram post around 3 a.m. that included a photograph of Mr. Usyk delivering a punch to Mr. Fury. “And in the end, all our opponents will be overcome.”

Ukrainian troops are currently engaged in fierce fighting to halt Russia’s grinding advance all along the front line, and there are fears that some key positions may soon fall. Russian troops recently advanced farther into Robotyne, a village in the south that was one of the rare successes of Ukraine’s failed counteroffensive last summer.

Faced with such grim prospects, many Ukrainians watched the match hoping that a win would lift their spirits.

“This victory is very good for raising our morale,” Valentyna Polishchuk, 54, said on Sunday in Kyiv, the capital. “Things are not good in our country, and this is at least something good.”