Serena Williams started so strong on Sunday that she looked to be in fourth gear. She downshifted from there, but did not stall for long on her way to the United States Open quarterfinals.

After winning the first set by 6-0 in 18 dazzling minutes, hitting 14 winners and only two unforced errors, Williams slipped before ultimately regaining control, prevailing, 6-0, 4-6, 6-3, over Kaia Kanepi.

Kanepi, a powerful Estonian veteran ranked 44th, had knocked out the top-seeded Simona Halep in the first round last Monday in two emphatic sets.

She could do little against Williams at the outset of their fourth-round match, but gained a foothold by breaking Williams in the opening game of the second set.