HONG KONG — Over the last year, A2, an Australian milk company, made an unexpected discovery: A sharp increase in domestic sales came from Chinese bulk buyers, not local mothers.

To bridge the 4,000-mile gap between its continent and the doorsteps of its newest customers, A2 began selling its products in two of China’s biggest online marketplaces — Tmall, part of Alibaba, and its rival JD.com. Six months later, A2 is part of the frenzy of discounting that is Singles Day, China’s annual shopping festival.

The commercially driven event began as a celebration of single people, held on a date that said it all, 11/11. But it has long since moved beyond the province of lonely shoppers and Chinese vendors. This year, a long list of foreign companies waited for the clock to hit midnight.

At stake were potentially billions of dollars in sales and access to the wallets of a growing Chinese middle class. This year, Alibaba sold $14.3 billion worth of products, surpassing last year’s total of $9.3 billion. JD.com and other Chinese e-commerce companies also got into the Singles Day action with their own deals.