Serena Williams upset in third round of Olympics singles

Defending champion Serena Williams is out of the Rio de Janeiro Olympics after a stunning loss in the third round of singles.

The No. 1-seeded American was beaten 6-4, 6-3 on Tuesday night by Elina Svitolina of Ukraine.

Williams made 37 unforced errors, including eight double-faults — five in one game.

She entered the night with a 4-0 record against Svitolina.

But all those mistakes added up, and now the 2012 London gold medalist in singles and doubles is finished in Rio.

She and sister Venus lost in the first round of doubles.