As the Belgian wild-card entry waited to play the next point, Williams, who had already been handed a code violation for racquet abuse, vented her frustration.
She has bounced back from a shock second-round loss to British teenager Laura Robson at January's Australian Open and a surprise defeat by French wild-card entry Kristina Mladenovic at the Paris Open, winning both matches as the Czechs thrashed Australia in the first match of their Fed Cup title defense.