The announcement is likely to add further volatility to the stock market, where US -technology stocks have helped drive a major correction in recent months. Cook's letter noted that services revenue was $10.8 billion in the quarter and the business should double from 2016 to 2020.
Further, Apple expects a gross margin of about 38 percent, along with operating expenses of approximately $8.7 billion ($11.8 billion), according to a press release from the company.
But the uptick didn't continue.
CEO Tim Cook told investors that come January 29, the phone-flinger won't hit the revenue figures it said it would reach, and would in fact see its first year-over-year decline since 2016. Blaming mostly macro headwinds, he failed to admit the launch of new iPhones haven't successfully lured consumers and acknowledge growing competition from domestic players.
"While Greater China and other emerging markets accounted for the vast majority of the year-over-year iPhone revenue decline, in some developed markets, iPhone upgrades also were not as strong as we thought they would be", the statement said.
However, he admitted that sales in more mature economies had also been disappointing.
"While we anticipated some challenges in key emerging markets, we did not foresee the magnitude of the economic deceleration, particularly in Greater China", Cook wrote.
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Those reasons don't fully explain iPhones' declining market share in China, especially in a quarter when it released new models, which used to be welcomed by Chinese consumers with much fanfare. "And market data has shown that the contraction in Greater China's smartphone market has been particularly sharp".
Economic growth in China slowed in 2018, and signs point to further deceleration this year.
A shock revenue warning from Apple has stoked fresh fears about trade tensions and spending in the world's second largest economy. For its iPhone business, Apple needs a new approach now that it exhausted its strategy of slowly knocking up the average price of the iPhone over the years.
Still, some local players managed to grow against the trend and they may be eating into Apple's market share.
Shares of the iPhone maker cratered almost 8 percent in extended trading Wednesday - knocking some $50 billion off the market cap after the company took the unusual step of cutting its revenue guidance.
People walk past an Apple store in Beijing on Dec 11. We'll know more as brands like Louis Vuitton, Chanel and Burberry report their quarters.
In December, Bloomberg News reported the company was facing a "fire drill" to boost iPhone sales, which led the company to aggressively market the product at lower than actual prices.