Tomic, who was battling with a wrist injury, accused Kyrgios of "faking" an illness in Sunday's four-set loss to world No.11 Jack Isner during a courtside chat with coach Lleyton Hewitt.
Kyrgios later fired back on Twitter, telling Tomic he would not have his back anytime soon.
However Masur defended Kyrgios' absence from the tie after he was ruled out by medical staff on Thursday.
"I didn't see him but the word was that he had a virus and his actual mouth was ulcerated and the guy was pretty fatigued," he said.
Masur's claims were backed up by Kyrgios' Davis Cup teammate John Peers, who said he was "a little" disappointed by Tomic's mid-match comments.
"But I think Bernie just got a little heated in the moment but it all calmed itself down," he said.
"I think Lleyton made the right decision to begin with. Nick came down to do a fitness test and according to medical staff didn't get through it.
"So Lleyton had to make the tough decision and go with the strongest team we could put out on the field that was healthy to play."
-AAP
