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.
