US Open 2018: Serena Williams beats Kaia Kanepi to reach quarter-finals

Six-time winner Serena Williams reached the US Open quarter-finals with a 6-0 4-6 6-3 victory over Kaia Kanepi.

Williams, 36, took the first set in only 18 minutes as she converted all three break points she earned and did not drop a point on her first serve.

But Estonia's Kanepi responded, displaying some of the shot-making that helped her beat world number one Simona Halep in the first round.

Williams broke early in the third set as she completed her win in 97 minutes.

The 23-time Grand Slam singles champion will now play Czech eighth seed Karolina Pliskova.

And the American warned her rivals that her fitness has improved since reaching the Wimbledon final in July.

She returned to competitive tennis in March after a year out during which she gave birth to her first child.

"It wasn't an easy match at all," said Williams, who last won the US Open title in 2014. "Kaia knows how to play and has a lot of big wins. She has had a great career.

"We always have tough matches, so I am just happy to get through. She was hitting such big returns so I wanted to hit bigger second serves.

"I like it when players challenge me to do that. I think what I have improved most is my fitness. Coming back from having a baby is really hard."