(Reuters) - Lawyers for Alphabet's Google and "Fortnite" maker Epic Games are set to square off before a U.S. appeals court in California on Monday, as Google tries to undo a jury verdict and a judge's order forcing it to revamp its app store.

Google has argued in court filings to the San Francisco-based 9th U.S. Circuit Court of Appeals that a trial judge made legal errors in the antitrust case that unfairly benefited Epic Games.

Epic accused Google in a 2020 lawsuit of monopolizing how consumers access apps on Android devices and how they pay for transactions within apps. The Cary, North Carolina-based company convinced a San Francisco jury in 2023 that Google illegally stifled competition.

U.S. District Judge James Donato ordered Google in October to restore competition by allowing users to download rival app stores within its Play store and by making Play's app catalog available to those competitors, among other reforms. 

The order is on hold as the 9th Circuit weighs Google's appeal. 

The tech giant has argued that its Play store competes fiercely with Apple's ( AAPL ) App Store, and that Donato unfairly allowed Epic to tell jurors that Google and Apple ( AAPL ) are not competitors for app distribution and in-app payments. 

Google also said in its appeal that Donato was wrong to issue an order affecting users and developers nationwide, not just Epic. It said the judge was acting as "a central planner responsible for product design."

Epic has asked the 9th Circuit to reject those arguments and accused Google of a "years-long strategy to suppress competition among app stores and payment solutions."

In a statement, Epic said it will "fight to ensure that the jury's verdict and the court's injunction are upheld and Google is held to account for its anticompetitive behavior."

Microsoft ( MSFT ) filed a brief backing Epic, as did the U.S. Justice Department and Federal Trade Commission.

The 9th Circuit could issue a ruling later in the year. Its decision can be appealed to the U.S. Supreme Court.