Britain's Kyle Edmund went out of the men's singles with a straight-set defeat by Japan's Taro Daniel in Rio.
Daniel, ranked 117, trailed 0-3 in the first set and 2-5 in the second, but won 6-4 7-5 and goes on to face Juan Martin del Potro or Joao Sousa.
Fellow Briton Andy Murray continues the defence of his singles title against Juan Monaco of Argentina on Tuesday.
Johanna Konta and Heather Watson play singles later on Monday, before teaming up in round two of the doubles.
"I am disappointed to lose," said Edmund.
"I know what I could have done better, I just wasn't able to quite do it on the court and problem solve during the match.