New York: Serena Williams booked her spot in the US Open quarter-finals on Sunday, and said the biggest challenge of her post-pregnancy comeback was regaining fitness.

"Coming back from a baby is really hard, harder than I thought," said Williams — drawing a cheer from the crowd on Arthur Ashe Stadium who had watched her 6-0, 4-6, 6-3 victory over Estonian Kaia Kanepi. Her victory came a day after daughter Olympia turned one.