You might think that Apple's (AAPL - Get Report) weak iPhone sales growth would be good news for rival Samsung (SSNLF , but Apple is also a big buyer of Samsung's advanced screen technology. As a result, concerns about slowing iPhone sales have prompted a Samsung analyst to reduce his estimates on the South Korean tech and consumer electronics giant. 

"Weak smartphone industry demand is certainly pressuring Samsung's Display business unit, particularly for OLED panel," wrote Susquehanna Financial analyst Mehdi Hosseini in a note out Monday morning. "In fact, our checks suggest Apple has dramatically cut back on the purchase of flexible OLED panels, as they try to keep channel inventory to a minimum." Hosseini notes that Apple is Samsung's largest customer for flexible OLED displays.

He is now reducing estimates on Samsung "to account for weaker memory (no surprise) and Display (a surprise to us)," he wrote.

Susquehanna's tracking of iPhone builds shows that 37 million phones have been produced in the March quarter to date (March 25), which is below the initial forecast of 40 million. The 37 million is also down 50% quarter-over-quarter. As a result, Hosseini has reduced his revenue estimates for Samsung's display business to reflect a 50% quarter-over-quarter decline, worse than the initially expected 30% to 40% decline. He expects first-half 2019 OLED display shipments to decline 20% against the first half of 2018. 

Hosseini's earnings per share estimate for all of 2019 is now $3.16, down from his initial call of $3.41. 

While many are focused on services as the next driver of Apple's growth, smartphone sales still comprise the majority of Apple's revenue, and they are one of the main drivers of the semiconductor industry. 

TheStreet will be hosting a live blog breaking down Apple's event at 1 p.m. ET on March 25. Please check our home page then for more details.

And while Samsung is seeing its display sales get hit, it is also emerging as a dangerous threat to Apple's iPhone sales. "One bright spot for Samsung is GS10, its flagship smartphone, tracking to 10-12M unit shipments in the March quarter, with a forecast for another 10-12M shipments in the June quarter, as demand is actually tracking a tad better than expectations," Hosseini said. 

In February, Wedbush securities Dan Ives told TheStreet, "on the high end of the market, Samsung is not being shy on pricing, and they view it as an opportunity to gain share on Apple," referring to Samsung's release of its pricey new foldable smartphone with three screens that will start at $1,980. "Samsung's gains are Apple's losses." 

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