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SHANGHAI (Reuters) - The U.S. head of Nio Inc NIO.N, an electric carmaker seen as one of the main Chinese rivals to Tesla Inc TSLA.O, will leave the company, the firm said in a filing on Friday, the first major management departure since its September IPO.

Padmasree Warrior, chief executive officer of NIO USA and global chief development officer, will resign from her posts on Dec. 17 for “personal interests”, the company said.

Warrior joined Nio, previously called NextEV, in 2015, and led the company along with founder and CEO William Li to become one of China’s most legitimate challengers in the global race to develop electric vehicles.

Before joining Nio, Warrior was chief technology and strategy officer at Cisco Systems Inc CSCO.O and chief technology officer at Motorola.

China is the world’s largest and fast-growing market for new-energy vehicles (NEVs), a category comprising electric battery cars and plug-in electric hybrids.

NEV sales in the first 10 months of 2018 came in at 860,000 vehicles, up 75.6 percent year-on-year.

Competition is, however, rising as Beijing looks to rein in subsidies that led to a huge array of EV contenders entering the market. Some of those are now being weeded out.

Tesla itself is building a car manufacturing plant in Shanghai to bolster its presence in the market and reduce its prices to become more competitive.

Nio’s shares - up strongly this year on robust revenue growth and bullish views from some analysts - dropped more than 4 percent in U.S. trading on Thursday.