Longtime Apple executive Phil Schiller has acknowledged a court-ordered makeover of the U.S. payment system in its iPhone app store hasn’t done much to increase competition — a shortcoming that could result in a federal judge demanding more changes.

Apple CEO Tim Cook described the company’s ironclad control over its mobile app store as the best way to serve and protect iPhone users, but faced tough questions about competition issues from a judge while testifying Friday about allegations that he oversees an illegal monopoly.

Apple CEO Tim Cook will take the witness stand on Friday in a high-stakes courtroom battle.