Some were skeptical that China would follow through on its pledges of openness. “People will say about the Boao speech: ‘Show me.’ We heard this in Davos last year,” Joerg Wuttke, former president of the European Chamber of Commerce in Beijing, said in referring to Mr. Xi’s 2017 speech at the World Economic Forum.

In a briefing, the White House press secretary, Sarah Huckabee Sanders, said that the administration was encouraged by Mr. Xi’s words but that it wanted to see concrete actions from China. In the meantime, she said it would keep moving forward with its plans to impose tariffs.

At a time when the United States’ policies have threatened to upset the stability of the world order, China’s growing confidence and its verbal support of global trade rules offer other countries a potentially appealing alternative to Mr. Trump’s rhetoric.

Mr. Xi spoke just days after the United States and China exchanged tit-for-tat tariff threats that have ignited worries of a global trade war. Trump administration officials have accused China of forcing foreign companies doing business here to give up trade secrets as part of Beijing’s effort to retool the Chinese economy and create companies that can compete with American rivals.

On trade, China has tried to project a balanced tone. It retaliated quickly last week after the United States detailed proposed tariffs it wanted to levy on about $50 billion in Chinese-made goods, saying it would match Washington’s efforts dollar for dollar. At the same time, Chinese officials have said they want to avoid a trade war and negotiate.

On Tuesday, Mr. Xi appeared to have given Mr. Trump a concession by pledging to “significantly” lower tariffs on imported automobiles by the end of the year. Just hours before, Mr. Trump had taken to Twitter to complain about China’s 25 percent tax on imported automobiles.