(Reuters) -Gorilla Glass maker Corning on Tuesday forecast fourth-quarter sales above analysts' estimates, but revenue from its largest business unit that makes fiber optic products fell short of expectations.

Shares of the Apple ( AAPL ) supplier fell more than 7% in premarket trading after the results, though the stock is still up about 88% so far this year.

The company's optical communications division, which accounts for nearly 40% of total revenue, recorded net sales of $1.65 billion for the third quarter, below analysts' average estimate of $1.73 billion, according to LSEG data.

U.S. optical fiber companies Corning, OFS-Felite and Drake Communications have been pressured by slower network investment in China, where authorities in September imposed a 37.9% anti-dumping tariff on imports of Corning's optical fiber products and other cable providers.

The tariff move added to U.S.-China trade tensions, further complicating operations for American technology and manufacturing companies exposed to Chinese demand.

The results come even as Corning deepens its partnership with Apple ( AAPL ), which in September said all cover glass for iPhones and Apple Watches will now be made in Kentucky using Corning's proprietary glass technology.

Its core sales in the quarter rose 14% to $4.27 billion, compared with estimates of $4.23 billion, according to data compiled by LSEG.

Corning said it expects fourth-quarter core sales to be around $4.35 billion, compared with the average estimate of $4.26 billion.

The company forecast quarterly core earnings per share between 68 cents and 72 cents, compared with estimates of 67 cents.

(Reporting by Kritika Lamba in Bengaluru; Editing by Saumyadeb Chakrabarty)