World number one Serena Williams who was beaten by Jana Cepelova in the second round of the Family Circle Cup in South Carolina is still bitter about her ouster.

The defending champion, 32, shockingly 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 for me, but she (Cepelova) 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.

However, Serena’s sister 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.