New details have emerged in the ongoing antitrust case between Google and the U.S. Justice Department, revealing that Microsoft offered to sell its Bing search engine to Apple in 2018 but was rejected over concerns about Bing’s quality.

The revelation came in a court filing (PDF file) earlier this month that was unsealed on Friday as part of Google’s legal battle against claims it holds an unlawful monopoly in the online search advertising market.

The filing shows that Microsoft has repeatedly tried to make Bing the default search option on Apple’s Safari browser since 2009, only to be denied each time over issues with Bing’s search results compared to Google’s.

“In each instance, Apple took a hard look at the relative quality of Bing versus Google and concluded that Google was the superior default choice for its Safari users. That is competition,” Google wrote.

Microsoft’s latest attempt came in 2018, when it proposed either selling Bing outright to Apple or establishing a joint venture, according to the filing. But Apple executive Eddy Cue dismissed the offer, saying “Microsoft search quality, their investment in search, everything was not significant at all.”

“And so everything was lower. So the search quality itself wasn’t as good. They weren’t investing at any level comparable to Google or to what Microsoft could invest in. And their advertising organization and how they monetize was not very good either,” Cue continued.

Google is aiming to prove through documents like this one that it competes fairly with rivals in the search market and that exclusivity deals to be the default search engine on various devices and browsers – which Google spends over $26 billion a year to maintain – are normal business agreements not indicative of monopolistic practices.

But the Justice Department counters that Microsoft has spent close to $100 billion on developing Bing over the past 20 years and that Apple rebuffed Microsoft’s latest overture mainly because it would lose out on billions in annual default search payments from Google.

Today, Bing holds just a 3% share of the global search engine market, compared to Google’s overwhelming dominance. And Microsoft’s overall search advertising revenue of $3.2 billion last quarter pales in comparison to Google’s $48 billion.

Yet Microsoft CEO Satya Nadella testified in October that he has continually tried coaxing Apple into making Bing the default on iPhones and other devices. And reports from last fall indicated Microsoft and Apple held talks as recently as 2020 about a possible Bing sale.

The legal wrangling between two of tech’s biggest giants provides a rare glimpse into the behind-the-scenes business deals that impact what services and products consumers have access to. And the stakes are extremely high, with the future of search and online advertising hanging in the balance.