** Apple AAPL.O shares trading flat after the iPhone maker submitted a legal challenge to the EU order to open its closed ecosystem to rivals
** EU's Digital Markets Act aims to curb Big Tech's power; AAPL claims demands stifle innovation
** Co argues interoperability rules pose privacy risks, target AAPL exclusively, affecting user experience
** Legal battle expected to last years; AAPL must comply with EU order in meantime
** AAPL stock down 20% YTD, underperforming Nasdaq .IXIC which fell 1%
(Reporting by Noel Randewich)
((noel.randewich@thomsonreuters.com))