The Ukrainian boxer Oleksandr Usyk became the world’s undisputed heavyweight champion on Sunday. The victory has lifted morale in a country struggling to contain Russian advances on the battlefield.

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.”

Maj. Ilya Yevlash, a spokesman for the Ukrainian Air Force, said that “without a doubt,” Mr. Usyk’s victory “had an impact on the morale of all Ukrainians, and we now desperately need such victories!”

Many Ukrainian public figures, including former President Petro O. Poroshenko and the mayor of Kyiv, Vitali Klitschko, himself a former heavyweight world champion, joined in the celebrations, portraying the victory as evidence that Ukraine was capable of defeating a strong opponent.

On Sunday, another Ukrainian boxer, Denys Berinchyk, won a lightweight world title, adding to the festive mood.

“We have to do everything possible to make sure that our next victory is our great victory,” Mr. Poroshenko wrote on Facebook.

Since the Russian invasion began in February 2022, Mr. Usyk has used his appearances in the ring to draw attention to Ukraine’s fight on the battlefield.

He has often worn a traditional Ukrainian hairstyle, consisting of a long lock of hair growing from the top of a shaved head. On Sunday, he celebrated his victory by waving a Ukrainian flag bearing the symbol of the country’s military intelligence agency.

Mr. Usyk is also an active fund-raiser for the Ukrainian Army. His charity, the Usyk Foundation, has raised $740,000 so far, according to its website.

But the Ukrainian boxer has also been criticized in the past for his seemingly ambiguous stance on Russia. A native of Crimea, the peninsula illegally annexed by Russia in 2014, Mr. Usyk has been accused of having ties to the Russian Orthodox Church of the Moscow Patriarchate, which has strongly supported Moscow’s war in Ukraine.

With his victory on Sunday, Mr. Usyk became the first undisputed heavyweight champion for nearly a quarter-century. He emerged triumphant after battling his way through a grueling 12-round fight against Mr. Fury in Riyadh, Saudi Arabia.

The parallels with Ukraine’s exhausting war against Russia were not lost on Ukrainians. “Here, every blow can change the course of the battle,” Major Yevlash said.

Several Ukrainians on Sunday said they hoped that Mr. Usyk’s victory would show the world that Ukraine was still capable of winning.

“I am very proud that Oleksandr has won this award. It is very important,” said Fedir Ilarionov, of Kyiv. He was standing in St. Michael’s Square, in the center of the capital, where the Ukrainian authorities have exhibited Russian armored vehicles destroyed and captured on the battlefield.

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Pavlo Velychko, a Ukrainian lieutenant who defends Ukraine’s northeastern border, struck a more realistic tone.

“It’s great, positive news for Ukraine,” he said in a telephone interview. “But our most important victories happen every day on the battlefield. That is where the attention of the Ukrainian nation and the world should be focused.”