Amazon's (NASDAQ:AMZN) share of the U.S. e-commerce market will rise to 47% of total sales this year, according to estimates from eMarketer. The four next largest e-tailers could claim a combined market share of just 16.2%.

Chart showing market share of top e-commerce platforms in the U.S.

Data source: eMarketer. Chart by author.

eBay (NASDAQ:EBAY) is the only top five e-tailer that is expected to lose market share this year, possibly due to tougher competition from Amazon. Last October, eBay sued Amazon for allegedly poaching its high-value sellers.

Walmart's (NYSE:WMT) U.S. e-commerce sales surged 43% year over year last quarter, thanks to the aggressive expansion of its digital platforms and delivery options, as well as acquisitions of smaller e-tailers. Walmart is also turning its brick-and-mortar stores into a fulfillment network to counter Amazon. However, its e-commerce revenue still only accounts for a small percentage of its total sales.

Meanwhile, Apple's market share is expected to be flat, while The Home Depot's should grow a little. The home improvement retailer posted an impressive 28% year-over-year jump in digital sales during the third quarter.

The competition is heating up, but these smaller rivals won't throttle Amazon's growth anytime soon. The best-case scenario, as demonstrated by Walmart, is to roll with the punches and leverage core strengths (like brick-and-mortar stores) to stay competitive.