HONG KONG — In China, the smartphone battle used to be Samsung versus Apple. But not anymore.

Over the summer, a Chinese company, Xiaomi, took the No. 1 position in China’s competitive market and became the world’s third-largest phone maker in the process.

Founded in 2010 as a lean start-up to sell smartly designed phones at cheap prices over the Internet, Xiaomi was decidedly late to the game. Its first handset came out around the time of the iPhone 4S.

But a clever social media strategy and a business plan that emphasized selling services that work on the phone helped Xiaomi build frenzied support from young and trendy Chinese.

With people in China expected to buy 500 million smartphones in 2015 — more than three times as many as will be sold in the United States, according to the research firm IDC — Xiaomi is poised to cement its place as one of the most powerful phone makers in the world’s most important market.