Apple is bringing out the big guns as the company heads toward wrapping up its defense at a antitrust case attracted by Epic Games.
Epic attorneys were expected to spend several hours grilling Cook on the rack.
The questioning may focus on Cook's strategies since accepting the CEO job almost a decade back, just a few months before Steve Jobs died of cancer in October 2011.
Epic Games, maker of the popular video game Fortnite, is trying to prove that the store has morphed into a price-gouging vehicle that not only reaps a 15 percent to 30% commission from in-app transactions, but blocks apps from offering other payment options.
When Epic tried to evade the commissions using an alternate payment system in Fortnite past August, Apple ousted it from the app shop to set up a legal showdown that would make it lower its fees.
Apple fiercely defends the commissions as a fair way for program makers to help pay for innovations and safety controllers, but Epic's attorneys see it as an excuse for maintaining the program store's huge profits.
The App Store has become a key revenue source for Apple, helping to power the organization to $57 billion profit in its last financial year. It ranks among Apple's biggest successes during Cook's reign. Since starting with only 500 programs in 2008 the store has ballooned to 1.8 million apps,
Apple's ironclad control over the App Store is currently under investigation by regulators and lawmakers in Europe and the U.S.