Steve Jobs today faced having to interrupt his sick leave to testify about alleged anti-competitive practices involving his iPod and iTunes music-download services.
A California district court has ordered Jobs to answer questions for two hours about Apple's treatment of rival firm RealNetworks and its Harmony technology.
Judge Howard Lloyd said: "The court finds that Jobs has unique, non-repetitive, first-hand knowledge about the issues at the centre of the dispute."
The technology giant faces accusations that it stopped songs from RealNetworks' own music store from being played on Apple's iPod player.
According to the lawsuit, RealNetworks' Harmony technology initially worked on iPod when it was first rolled out in July 2004.
But the iPod maker announced just five days later that it would update its encryption software to block RealNetworks.
Instead Apple stuck to its own proprietary software, called FairPlay, which allegedly meant only music tracks bought from iTunes worked on iPod. The iTunes store went on to enjoy huge success, overtaking Wal-Mart to become the biggest music retailer in America in 2008.
iTunes customer Thomas Slattery began his court case against Apple in 2005, claiming the company illegally limited consumer choice. Lawyer Bonny Sweeney, acting for Slattery and other consumers, said: "Apple took steps that excluded potential rivals from the market and that served to maintain its monopoly in the digital portable player market."
Apple declined to comment.
This is not the first time the maker of the iPhone and iPad has faced allegations over its dominance of the music market. The computing giant cut the prices of iTunes songs sold in Britain in 2008 after a European Union competition inquiry.
Since 2009, all digital music files sold on iTunes have been sold without proprietary software.
Jobs has been on medical leave since January after previously fighting pancreatic cancer and having a liver transplant.
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