A stock market plunge last summer, and a messy currency devaluation that followed, had faded from global view. In the relative calm, he seemed to usher in a new era of economic management, promising policy coordination at the highest levels to prevent another bout of turmoil.

Less than three weeks later, his plans have been derailed as China’s stock market and currency once again rattle investors around the world. The latest rout sets up a challenge for Mr. Xi, who has positioned himself as the master of the country’s economy.

At every turn, the president’s efforts to manage the economy, market and currency have been undercut by global headwinds and haphazard policy making. Three initiatives this week, involving currency depreciation and two sets of stock market rules, have been particularly discordant. All three were hastily suspended after China’s stock market plunged on Thursday morning.

He also cannot move forward on the bolder actions needed to head off a more serious economic slump, such as forcing hopelessly indebted state-owned enterprises to stop borrowing money and shut down. Otherwise, he risks further eroding short-term confidence and growth, which have depended heavily on this borrow-and-spend mentality, and mass layoffs could follow.

Mr. Xi’s options are also more limited than in the past. He and his aides engineered the elevation of the renminbi to the ranks of the world’s leading currencies, a status bestowed by the International Monetary Fund in November. But in doing so, he gave up some control, allowing market forces to play a bigger role.

In the last couple of years, China had begun allowing, even encouraging, companies and people to invest more of their wealth overseas. Doing so helped reduce deflationary pressures at home from chronic overinvestment and overcapacity, and increased China’s influence around the world.