Forward Dragan Mrdja scored twice for the lively Serbs, by far the hungrier of the two South Africa-bound teams in a match for which neither coach had a full-strength side available.
"We did exactly what we knew we couldn't afford to by conceding an early goal," Japan captain Yuji Nakazawa told Reuters.
"We know we're going to face even more dangerous players at the World Cup in June. We just have to take this loss on the chin and move forwards."
"There was plenty of quality tonight for (Serbia coach Radomir) Antic to consider when he chooses his World Cup squad," Serbia assistant coach Radovan Curcic told reporters.
"The players showed they will soon be competing for places with the senior players."
"I honestly thought we would be better than that," a glum Okada told reporters. "There wasn't the motivation there should be when you wear the Japan shirt. For that I apologise.
"We had 70 percent of the possession but did nothing with it. It has underlined how reliant we are on our top players."
"We were poor. We had no rhythm," said Japan playmaker Shunsuke Nakamura. "We need to go back to basics."
