Apple Inc has pumped $1 billion into ride-hailing service Didi Chuxing, the main competitor in China of Uber Technology, in an unusual move that Apple CEO Tim Cook said will help it "learn more about certain segments of the China market".

The investment, announced on Friday, is the largest single investment that Didi has received. It also is a rare one for Apple, since the US tech giant tends to buy out companies rather than inject capital into them.

Apple declined to elaborate on the intentions of the deal, but Cook told Reuters that he saw opportunities for Apple and Didi to collaborate in the future.

"We are making the investment for a number of strategic reasons, including a chance to learn more about certain segments of the China market," he said. "Of course, we believe it will deliver a strong return for our invested capital over time as well."

Didi, which delivers more than 11 million rides a day via its platform, has an estimated valuation of $25 billion. The $1 billion investment makes Apple a strategic partner joining other Didi investors such as China's internet giants Alibaba Group and Tencent Holdings.

The investment comes at a time when Apple is seeing its sales tumble in China, its second-largest market.

"Apple seems to be running out of gas on its device sales, witnessed by its stock performance in recent weeks," said Travis Wu, research director at the Beijing office of multinational consultancy Forrester Inc.

"They must look for new ways to invest and expand, and they do have deep pockets."

Wu said Apple has been building its ecosystem in the auto industry with the rumored Apple Car and its CarPlay infotainment system. China is the world's largest automobile market, and Didi is the dominant player in China's ride-sharing sector, which offers a great test field for Apple.

"Working with Didi can help them further understand the Chinese market, technologies and business models," he said.

James Yan, research director at Counterpoint Technology Market Research, said that by teaming up with Didi, Apple is also expected to boost the loyalty of Chinese users. "For example, Apple can integrate Didi with its existing services, such as map and Siri. Asking Siri to help hail a ride can certainly help increase user experience in China," he said.

He added that the company's payment service, Apple Pay, may also get a boost in China as an increasing number of people now pay by smartphone after booking a ride on Didi.

Cheng Wei, founder and CEO of Didi, said in a statement that the endorsement from Apple is an enormous encouragement and inspiration for the 4-year-old firm.