has fundamentally changed since the iPhone was first introduced in 2007. Back then, Apple marketed the device as a lifestyle accessory, but as smartphones have become ubiquitous, consumers' focus has increasingly shifted to the features on offer.

"If you look at Huawei phones, or Xiaomi phones, it's like 'Wow they're really good'," said John Butler, a Bloomberg Intelligence analyst. "They've got great battery life, the screens are really sharp, the features are great."

Apple more than doubled its Chinese revenue between 2013 and 2015 to $59 billion, expanding aggressively: it had 35 stores in the region by the end of March, up from 21 a year earlier, and aimed to add another five by the end of June. Apple has made efforts to remain on good terms with the Chinesegovernment, including a visit by Mr. Cook in May that coincided with a $1 billion investment in the country's biggest car-sharing service, Didi Chuxing Technology.

And yet sales in Greater China -- which comprises the mainland, Taiwan and Hong Kong -- fell 26% in the fiscal second quarter, accounting in large part for Apple's 13% sales tumble. Analysts expect total revenue to decline by a further 15% in the three months through June compared with a year earlier.

Mr. Cook attributed the fall primarily to the strength of the Hong Kong dollar, which is pegged to its American cousin and made it more expensive for tourists visiting the former British colony to shop. Excluding currency effects, sales in mainland China still fell 7%.

"Apple is expecting growth to come from the