Russian Olga Puchkova upset top-seeded Venus Williams 4-6 6-4 7-5 to reach the final of the Brazil Tennis Cup in Florianopolis.
World No 109 Puchkova will now play the final of the hardcourt tournament in southern Brazil against Monica Niculescu of Romania, who easily defeated seventh-seeded Kristina Mladenovic of France 6-0 6-2.
Williams broke Puchkova's serve at 5-4 to win the first set and was up 2-0 in the second, but she then tired and Puchkova broke her late in the second and third sets to close out the match in two hours, 31 minutes.
American Williams was playing in her first tournament since losing to Maria Sharapova in the third round of the Australian Open in January.
"I gave my best but just couldn't play consistent enough today," Williams said. "She played so well, it was hard."
Meanwhile, Williams' sister Serena was receiving a crash course in golf fan etiquette in Florida.
World No 1 Williams tweeted that she was taken to task by an official at the Honda Classic in Palm Beach Gardens for taking a picture of Tiger Woods in action.
"Apparently u can't take pics. This security for mad and yelled at me," Williams tweeted. "I need to learn golf, didn't know I knew so little."