Serena Williams in action at the US Open [Source: US Open]

Six-time champion Serena Williams fought back from a slow start to beat fellow American Sloane Stephens and reach the last 16 of the US Open.

Third seed Williams, 38, was sluggish in the opening set before raising her intensity to win 2-6 6-2 6-2 against 26th seed Stephens in New York.

“It was intense, we always have some incredible matches,” said Williams.

Williams, aiming for record-equalling 24th Grand Slam title, will face Greek 15th seed Maria Sakkari in the last 16.

Sakkari, 25, beat Williams in the three sets at the Western and Southern Open last week.