By Steve Keating

RIO DE JANEIRO (Reuters) - The U.S. men's team extended their Olympic unbeaten run to 21 games with a tense 94-91 win over Serbia on Friday, but their uneven play hinted that a third straight gold might not be a fait accompli.

After opening the Rio Olympics with blowout wins over China and Venezuela, cracks are starting to show in the armor of a usually invincible U.S. team that have been put to the test against Serbia and Australia in their last two games.

While the U.S. (4-0) lead Group A with one preliminary game remaining against France on Sunday, they have been plagued by turnovers in their last two contests and been unable to showcase the dominance that has long been their trademark.

But despite some uneven play there was no panic in the U.S. camp as players treated the near upsets as learning experiences ahead of the knockout phase of the 12-team tournament.

"I'm not too concerned," said Paul George, who had 12 points in the win while Kyrie Irving led the U.S. with 15. "We're a new group and playing against an experienced offense, we're still trying to figure it out.

"It's good we're having two games like these where we have to kind of pull it together.

"Right now we we're just going through an adjustment period."

Coming off Wednesday's 98-88 win over Australia, the United States appeared to have learned their lesson as they got off to a fast start against Serbia.

The Americans opened with a 14-2 run highlighted by a one-handed slam dunk from DeMarcus Cousins that triggered a round of high-fives from the American bench and signaled that a rout was underway.

But sixth-ranked Serbia had other plans and denied the Americans a chance to put the game out of reach as they pulled to within nine points at halftime.

The tireless Serbs began to wear down the Americans in the second half with their ball movement and with less than a minute to play trailed by three and were in position to pull off the biggest upset of the tournament.

But with the crowd on their feet and two seconds on the clock Bogdan Bogdanovic's three-point attempt clanged off the rim allowing the U.S. to escape with the win while Serbia dropped to 1-3.

"Serbian mentality is we have to fight to the end no matter what the score," said center Miroslav Raduljica, who had 18 points for the Serbs. "This is how we play and how we are going to keep playing, not just this tournament but always as long as Serbia has a team."

In earlier Group A action, Australia (3-1) earned a 93-68 win over China (0-4).

(Editing by Frank Pingue)