The U.S. were tied 1-1 with the underdogs going into the second period of their group stage game after the Latvians successfully challenged two of their goals, a frustrating start for the expected gold medal contenders.
"The energy on the bench is terrific and they never got deflated," said Sullivan. "We just kept playing."
Defenceman Quinn Hughes had found the back of the net early in the contest but it was called back for offside. Another would-be goal, this time from JT Miller, was called off later for goaltender interference.
Sullivan said the latter call was a tougher pill to swallow.
"It's such a difficult call and that one was a close one for me... I don't know that I agreed with the call," he told reporters. "But it doesn't matter what I think."
NHL players have had to adapt their game in a short time frame after the league went on break just a week ago, with the sport's global governing body famously taking a harder line on physical contact during the Olympics than in the pros.
Players from the top-flight league are back in the Olympics for the first time since 2014.
"I do think, in international play, just in my experience, they're a little bit stricter with that stuff than they are in the NHL," said Sullivan.
The U.S. play Denmark and Latvia play Germany in Saturday's Group C action.