Google is by far the dominant player in search advertising in the U.S., but Amazon and others are eroding that lead. Amazon’s share of the search advertising market is expected to grow as Google’s shrinks over the next two years, according to estimates released Tuesday by research firm eMarketer.

Amazon seeing double-digit revenue growth. This year, eMarketer forecasts Amazon’s paid search revenues in the U.S. will increase nearly 30% over last year to $7.09 billion. (In 2018, Amazon generated more than $10 billion in ad revenues globally.) Its share is expected to grow from 12.9% in 2019 to 15.9% by 2021.

Among the top five search advertising companies in the U.S. market, Amazon is the only one expected to see an increase in market share in the coming years.

Google’s shrinking share. Google will retain its strong lead, but its market share is forecasted to drop from 73.1% in 2019 (representing $40.33 billion) to 70.5% by 2021.

Up-and-coming smaller players. Walmart, which is building out its own marketplace, Pinterest, Target and eBay are not broken out separately, but eMarketer says search share of each is increasing.

Why we should care. The growing propensity for users to start their product searches on Amazon is one part of what is igniting its search growth. The other piece fueling momentum is the company’s investment in the Amazon Advertising UI, API and added ad formats and inventory, as we reported coming into this year.

In reaction to competitive pressures, Google launched a new Shopping experience in the U.S. this month. It offers merchants omnichannel opportunities to sell on their own sites, in-store or via Google’s universal checkout. Signed-in users get personalized results based on their browsing and shopping histories. Several thousand retailers now participate in Google’s cost-per-sale checkout program. Google also says it has mapped more than 2 billion products to local retailers.

Whether the new Google Shopping will help reset the company’s U.S. market share trajectory remains to be seen.