Apple Inc.’s Chinese smartphone shipments plummeted an estimated 20 percent in 2018’s final quarter, underscoring the scale of the iPhone maker’s retreat in the world’s largest mobile device arena against local rivals like Huawei Technologies Co. The domestic market contracted 9.7 percent in the quarter, but Apple declined at about twice that pace, research firm IDC said in a report on Monday. Smartphone labels from Apple to Samsung Electronics Co. are contending with a plateauing global market after years of breakneck growth, as a lack of innovation discourages consumers from replacing devices as often as they used to. Chief Executive Officer Tim Cook has seen China as a key part of Apple’s strategy: last fiscal year the company generated almost $52 billion in revenue from Greater China, a region that includes Hong Kong. But with the country announcing its slowest economic growth since 2009, Apple said its sales fell 27 percent in the holiday quarter.