Apple boss Tim Cook has hinted it could lower iPhone prices in some places in an attempt to boost falling sales.
Revenue from the iPhone, responsible for most of the firm's profits, fell 15% in its latest financial quarter.
Overall the firm's revenue was down 5% from a year ago to about $84.3bn (£64.5bn).
The slowdown had been expected after the tech giant warned investors earlier this month that revenue would be about $84bn, lower than expected.
The firm had blamed the issues partly on an economic slowdown in China.
But chief executive Tim Cook said customers were are also struggling with the firm's high prices.
He said a strong dollar, which makes its products comparatively more expensive, had hurt its sales in emerging markets.
Mr Cook said the tech giant had started this month to re-price its phones to shield customers from the impact of currency fluctuations.
"What we have done in January in some locations and [for] some products is essentially absorb part or all of the foreign currency move as compared to last year," he said.
Still, executives said they expected the firm's challenges to continue.
Apple predicted revenue for the three months to 31 March of $55bn-$59bn - suggesting a drop of at least 3.4% year-on-year.
"The macroeconomic environment, particularly in emerging markets, will continue to be there," Luca Maestri, the firm's chief financial officer, said.
Apple's struggles are not unique. Global smart phone shipments contracted 5% in 2018, according to Canalys, a market analyst firm.
But the firm's share price has dropped by around one third since October, amid investor concerns that buyer appetite for iPhones is weakening.
Fears intensified after the firm said it would stop reporting the number of iPhones, iPads and Macs it sold each quarter.
However, Apple shares gained more than 4% in after-hours trade on Tuesday, as the firm proved more resilient than expected.
Quarterly sales revenue dropped by more than 25% in its Greater China region, which includes Hong Kong and Taiwan, compared to the year before.
Sales also slipped about 3% year-on-year in Europe.
But in the Americas - the firm's single biggest region - sales were up almost 5%.
Revenue from the services business also jumped 19% to a record $10.9bn in the quarter, which ended on the 31 December.