When China’s top leader, Xi Jinping, hosts President Vladimir V. Putin of Russia in China this week, it will be more than two years since the two autocratic leaders declared a “no limits” partnership to push back against what they consider American bullying and interference.

Growing challenges from the West have tested the limits of that partnership.

Mr. Xi is walking a narrowing tightrope, coming under increasing diplomatic and economic pressure to curtail Chinese support for Russia and its war in Ukraine. A tighter embrace of Mr. Putin now could further alienate Europe, a key trading partner, as Beijing seeks to improve its image in the West, and retain access for Chinese exports to help revitalize its sluggish economy.

“China sees Russia as an important strategic partner and wants to give Putin proper respect, but it also wants to maintain sound relations with Europe and the United States for economic reasons and beyond. It is a very difficult balancing act,” said Shen Dingli, a Shanghai-based international relations scholar.

Mr. Putin, for his part, may be testing Mr. Xi’s appetite for risk, as he tries to deter Western nations from more actively supporting Ukraine. Last week, while Mr. Xi was in France meeting President Emmanuel Macron, Mr. Putin ordered drills for the use of tactical nuclear weapons. The move was seen as the most explicit warning so far that Russia could potentially use battlefield nuclear weapons in the war, which Mr. Xi has explicitly drawn a line against.

The Russian leader is also likely to press Mr. Xi for more support to sustain his country’s isolated economy and its war machine in Ukraine.

Show of Unity and Strength

Mr. Putin has just celebrated his fifth inauguration as president, setting him up to become the longest-serving Russian leader in centuries if he serves his full term. And Mr. Xi has just returned from a trip to Europe where he was exalted in the pro-Russian states of Serbia and Hungary and wined and dined in France. He left the region without making any major concessions on trade or Ukraine.

Mr. Xi has met with Mr. Putin over 40 times, including virtually, more than any other leader. The two often exchange birthday greetings and refer to each other as an “old” or “dear” friend. More crucially, they also appear to see in each other a strategic partner in a great geopolitical rivalry and will likely use the talks to depict themselves as leaders of an alternative global system aimed at eroding American dominance.

“The goal is to demonstrate how closely China and Russia are standing next to each other,” said Yun Sun, the director of the China program at the Stimson Center in Washington.

But this solidarity with Russia makes China a target for Western pressure.

The United States asserts that Beijing, while not supplying lethal weapons, is still aiding the Kremlin’s war efforts by providing satellite intelligence, fighter jet parts, microchips and other dual-use equipment in addition to filling Moscow’s coffers as a top buyer of Russian oil. Washington has imposed sanctions on a slew of Chinese companies for links to the war, and threatened to blacklist Chinese financial institutions doing business with Russian firms.

Beijing’s tacit support for Moscow’s war in Ukraine has also hurt China’s standing with the European Union. In France, when confronted about the war, Mr. Xi bristled and said China was “not at the origin of this crisis, nor a party to it, nor a participant.”

China’s ‘Straddle’ May Be Working

Mr. Xi has made no suggestion that he would use his influence on Mr. Putin to bring the war to an end. And he may feel little need to do so.

China’s strategy of aligning with Russia while attempting to steady ties with the West at the same time, which some have described as a strategic straddle, may be paying off.

China’s relationship with the United States, which plummeted to multi-decade lows last year, is somewhat more stable now. And major European leaders continue to engage with Mr. Xi, including Chancellor Olaf Scholz of Germany, who brought business executives with him on a visit to Beijing last month.

The approach is winning more support at home for Mr. Xi. Chinese scholars and think tank analysts see the momentum on the battlefield shifting in Russia’s favor, said Evan S. Medeiros, a professor of Asian studies at Georgetown University.

“For Xi, the strategic straddle is working better than they could have imagined, and China has paid little cost for it,” he said.

Mr. Xi also needs Russia as a counterweight in his country’s rivalry with the United States, which plays out over U.S. support for Taiwan, China’s territorial claims in the South China Sea and access to cutting-edge technology. China and Russia have ramped up military drills in the East China Sea, placing pressure on Taiwan, the self-governed island Beijing claims as its territory.

“Even if the China-Russia relationship was not as close,” said Xiao Bin, a Beijing-based expert on China’s relations with Russia, “the political elites in the U.S. may not regard China as a strategic partner, but would keep viewing China as a potential threat, even an enemy.”

Putin’s Growing Dependence on China

Mr. Putin, however, runs the risk of becoming over-reliant on China to a degree that might have made Russian officials uncomfortable in the past. China has become Russia’s lifeline since the invasion of Ukraine, displacing the European Union as Russia’s largest trading partner.

Mr. Putin is still pursuing his own interests. His growing coziness with North Korea, which is supplying Russia with munitions, could result in both countries being less reliant on Beijing.

But amid its isolation from the West, the Kremlin has been left with little choice: Mr. Putin needs China to buy energy, to supply dual-use components such as computer chips to sustain his military, and to provide a currency with which to carry out foreign transactions.

Last year, some 89 percent of the “high-priority” imports necessary for Russian weapons production came from China, according to a customs data analysis by Nathaniel Sher, a researcher at the Carnegie Endowment for International Peace. Those include everything from machine tools used to build military equipment to optical devices, electronic sensors and telecommunications gear, the analysis found.

“It’s much more survival mode. You are in a war situation,” said Alexander Gabuev, director of the Carnegie Russia Eurasia Center and an expert in Sino-Russian relations.

For Mr. Putin, hedging against China “is a luxury he doesn’t have anymore,” he added.

Olivia Wang contributed reporting.