Paris: World No 1 Serena Williams blasted herself for a performance "verging on unprofessionalism" after she needed another three sets to get past compatriot Sloane Stephens in the fourth round of the French Open on Monday.


Serena Williams during her singles match against Sloane Stephens in Paris on Monday. Pic/Getty Images 

Williams beat the unseeded Stephens to reach the quarter-finals, but only after a two-hour encounter on Court Philippe Chatrier that finished 1-6, 7-5, 6-3.