World number one Serena Williams suffered a shock defeat by Jana Cepelova in the second round of the Family Circle Cup in South Carolina.
The defending champion, 32, lost 6-4 6-4 to the Slovak, 20, who is ranked 78 in the world.
Williams, who won the Sony Open last weekend, failed to recover after losing the first five games.
"Obviously things didn't go great, but she was a great counter-puncher and hit a lot of balls back," Williams said.
Cepelova had never previously beaten a top-15 player, but held her nerve to pull off the biggest win of her career.
Venus Williams, the 2004 champion, lost seven consecutive games at one stage, but reached round three with a 6-3 0-6 7-5 win over Barbora Zahlavova of the Czech Republic.
Sixth seed Eugenie Bouchard of Canada beat Russia's Alla Kudryavtseva 6-2 6-0, while Australian seventh seed Samantha Stosur had to battle to beat Yaroslava Shvedova of Russia 6-3 5-7 6-1.
Eighth seed Sorana Cirstea of Romania was knocked out by Brazil's Teliana Pereira who won 3-6 6-3 7-6.