Japan supplies “all the things used in China’s infrastructure expansion,” said Paul Sheard, the chief global economist at Standard & Poor’s, the credit ratings agency. Because of that, Japan now risks being hurt as that expansion slows.

China’s economy is hardly in ruins. It grew at an annualized rate of 6.9 percent in the third quarter, according to the official government estimate. While that was enviable by the standards of more advanced countries, it was the lowest rate since 2009, in the depths of the global financial crisis. And sectors that have been especially eager buyers of Japanese capital goods, like manufacturing, construction and mining, are in worse shape than the economy as a whole.

China overtook the United States as Japan’s top trading partner a decade ago. Roaring exports of things like mining trucks, textile looms and semiconductor fabricators have been a bright spot for Japanese industry, which has lost ground in more visible areas like consumer electronics. Diplomatic disputes that have divided Tokyo and Beijing have failed to halt a deepening interdependence in commerce.

China’s commercial relationship with Japan has been shifting, however. Rising labor costs in China have prompted many Japanese companies with factories there to spread their bets, turning to countries in Southeast Asia like Thailand and Indonesia.

In their place, Japanese retailers have poured into China to target newly wealthy consumers there. Convenience stores and clothing chains like Uniqlo have been expanding rapidly.

Tomokatsu Shiina, an architectural designer in Tokyo who regularly works in China, recently returned from Shanghai, where he was overseeing design for a Japanese discount shoe retailer’s first store in the city. “These are budget consumer items,” he said. “That’s where the activity is now.”

It is also in Japanese shopping districts like Ginza in Tokyo, where a standard amenity on package tours from China is an extra, empty suitcase, for filling with brand-name clothing, handbags and electronics. A tourism explosion brought 3.8 million mainland Chinese to Japan in the first nine months of this year, more than twice as many as visited in the same period last year. Their shopping habits have inspired a new Japanese word, “bakugai,” or “explosive buying.”