Google has plenty of competition, but when it comes to online search, Eric Schmidt worries most about Amazon.

“People don’t think of Amazon as search, but if you are looking for something to buy, you are more often than not looking for it on Amazon,” Google’s executive chairman said during a speech in Berlin this week.”They are obviously more focused on the commerce side of the equation, but at their roots, they are answering users’ questions and searches, just as we are.”

His remarks come days before Google GOOG reports third-quarter results on Thursday. Analysts predict strong growth, driven once again by robust advertising sales.

Take the longer view however, and Schmidt’s concerns about Amazon AMZN may have merit.

For years, Google has been the undisputed online ad king. Digital ads generate 90% of the company’s total revenues, according to RBC Capital Markets, revenues which in turn, Google has used to underwrite other initiatives including the futuristic research lab, Google X. Over a decade later, the ads remain effective because they’re so ingrained in Google search: Search for, say, Sony’s PlayStation 4 gaming console and PlayStation 4 deals pop up in those subsequent search results.

This is why Amazon’s swelling sales, largely driven by retail sales on its site, are worrisome for a kingpin like Google. Why bother performing a Google search for a PlayStation 4 when shoppers can effectively jump through fewer hoops by heading directly to Amazon.com, buying a new or used unit and having it in two days or less?

Indeed, Schmidt appears concerned some shoppers will bypass Google entirely. “If more people use Amazon as a shopping search engine, that’s a direct negative for Google,” says Mark Mahaney, managing director for RBC Capital Markets.

As two of the largest tech companies, Google and Amazon have butted heads several times this year.

In August, Amazon acquired Twitch, a video streaming service Google also reportedly looked at, for $1.1 billion. It also made headlines when reports claimed that Amazon, one of Google’s largest ad buyers, had its own ads product in the works. Amazon Sponsored Links, as it’s being called, would work like Google’s search ads.

In theory, Amazon-generated ads may prove even more “clickable” for shoppers than Google’s because they may be better targeted. Like Google, Amazon collects a huge amount of data about its users. But unlike Google, Amazon has a long history of what shoppers have bought. That information could be used by Amazon to make its ads more relevant than Google’s.

For its part, Google has made the ad experience more Amazon-like, introducing features like five-star customer reviews and expanding Google Express, its same-day delivery service. By fetching products at bricks and mortar stores, Google can slowly build up a shopping history on potentially millions of its users.

But a future in which Amazon dominates global online retailing remains unlikely. In a best-case scenario, Mahaney predicts Amazon could own up to 20% of online retail sales worldwide. Most people will continue to shop elsewhere, which leaves a huge opportunity for a cash-laden adversary like Google.

“Unless you’re Alibaba, the competition will still likely have to use Google as part of their strategy,” Mahaney said.

In other words? Don’t expect Amazon and Google’s intensifying rivalry to cool down any time soon.