SAN JOSE -- Attempting to undermine Apple's argument that Samsung should pay $2.2 billion for infringing the iPhone maker's patents, Samsung's chief defense attorney on Friday tried to portray one of Apple's chief witnesses in the companies' ongoing trial here as an expensive hired gun whose damage estimate was biased and unreliable.

Under questioning from Samsung attorney John Quinn in the patent case before the U.S. District Court, Chris Vellturo, Apple's damages expert, admitted he's worked on 15 different cases for the Cupertino company and Apple has paid him $2.3 million to work on this case alone.

"Apple has come back to you again and again when they needed your opinion in court," Quinn said, adding, "You're a professional witness for Apple."

Vellturo denied the charge, noting he'd also worked for Microsoft and Amazon. If he was a professional witness for Apple, "I wouldn't be working with their very direct competitors," Vellturo said.

In previous questioning from Apple's attorney, Vellturo said his damage estimate was based on three primary factors: a calculated "reasonable" royalty rate that Apple would have agreed to charge Samsung for the patents at issue in the case, sales that Apple lost due to the Samsung products that infringed on its patents and the amount of money Samsung gained from those products after it was notified of the infringement and before it was able to put in place workarounds to those patents.

Quinn, though, grilled Vellturo about whether he included in his calculation the troubles that Apple was facing in the market. He brought up multiple documents, many introduced earlier in the case, that indicated that starting around 2011 Apple was concerned about losing market share to Samsung but attributed its problems to factors other than Samsung infringing on its patents.

Quinn asked if Apple should have taken into account that it "didn't have what customers wanted."

"Yes," Vellturo responded. "And I do."

Under further questioning, Vellturo acknowledged his calculations of damages were largely derived from the work of another Apple expert -- MIT market professor John Hauser. Hauser had testified about the value of Apple's patents and how much extra consumers would pay for a device that included them.

Quinn suggested that because Hauser only gave one set of values for those patents, jurors had only the choice between agreeing with that value or rejecting it completely and valuing the patents at $0.

Vellturo scoffed at that notion.

"Apple would not grant this for nothing to a chief competitor," he said. "That's why what you're proposing doesn't make sense."

Apple concluded its portion of the case after Vellturo's turn on the stand. Samsung's first witness was Google engineer Hiroshi Lockheimer, who testified about the development of Android. The case will resume on Monday with more witnesses from Samsung.

Contact Troy Wolverton at 408-840-4285. Follow him at Twitter.com/troywolv.