The judge chairing Microsoft's appeal against a European Commission antitrust ruling has asked why the firm had to give technical codes to other firms.
Microsoft rejects claims it has prevented competition
The five-day hearing stems from Microsoft's rejection of a landmark ruling by the Commission in 2004.
The firm was fined 497m euros ($613m; £344m) after a ruling it abused its dominance by muscling out rivals.
Now the judge has asked the Commission why Microsoft was also forced to share "commercial information" with rivals.
As part of the 2004 decision, the Commission said Microsoft should give competitors technical information to allow them to make server software that works smoothly with Microsoft's Windows, used by 95% of the world's PCs.
The Commission said without this information, rivals cannot compete with viable, or interoperable, non-Microsoft software.
However, the software giant claims that this amounts to losing control of key technology that is protected by intellectual property rights.
It also argues it is being asked to reveal trade secrets that would dampen innovation.
Court of First Instance Judge John Cooke said: "The information which forms interoperability is hugely valuable commercial information."
He asked the Commission whether "competition rules require that (to) be taken away from Microsoft, conveying a huge commercial advantage".
Earlier in the day Microsoft's rivals claimed they have been forced to "play catch up" because the company has limited access to its software.
Speaking on the fourth day of the trial, Andrew Tridgell of an open source project Samba, said "we are more than 10 years behind."
The 2004 ruling also ordered Microsoft to change how it sells its Media Player software.
In court European Commission lawyer Anthony Whelan said the "super-dominant" position enjoyed by Microsoft prevents its rivals from innovating.
Mr Whelan said that even if Microsoft provided other software operators with the information to facilitate using their software on Microsoft operating systems, "an attack of the clones" was unlikely.
If the panel rules in favour of the Commission, it will confirm the EU's powers to tackle market dominance and also force Microsoft to alter the way it does business.
But if not, it could be seen as a major blow to the EU's ability to challenge anti-competitive behaviour.
While the hearing will end on Friday, a decision by European Court of First Instance is not expected for months and could take as much as a year.