Serena Williams finished with 18 aces and 47 total winners. A couple of loose points gifted Kanepi a break in the first game of the second set, however, and she couldn't get back on terms
New York: Six-time champion Serena Williams survived a spirited challenge from unseeded Kaia Kanepi to triumph 6-0, 4-6, 6-3 on Sunday and reach the quarter-finals of the US Open.
The 23-time Grand Slam champion looked set to romp home when she seized the first set from Kanepi in just 18 minutes.
But the hard-hitting Estonian, who took out world number one Simona Halep in the first round, settled down in the second set, shaking off the partisan crowd in a jammed Arthur Ashe Stadium to push Williams the rest of the way.
Also read: Serena downs sister Venus
"It wasn't an easy match at all," said Williams, who let out what she called a "Serena scream" after ripping a backhand cross court to hold serve in the opening game of the third set.
"Winning a big game and a very important game and a really tight game, I think it was just a relief," said Williams, who took control with a break in the next game and maintained that advantage until she fired a forehand winner on her first match point.
Williams finished with 18 aces and 47 total winners. A couple of loose points gifted Kanepi a break in the first game of the second set, however, and she couldn't get back on terms.
Williams double-faulted on break point to send Kanepi up 5-2. The American would win the next two games, but she couldn't find another break and after fighting off two set points saw Kanepi knot the match on her third chance.
"She's had a lot of big wins in her career. I was just happy to get through it to be honest," Williams said.
Williams booked a quarter-final date with Karolina Pliskova, the eighth-seeded Czech who beat Australian Ashleigh Barty 6-4, 6-4.
Pliskova defeated Williams in the semi-finals in New York in 2016.
Defending champion Sloane Stephens, the third seed, eased past Elise Mertens 6-3, 6-3 to book a quarter-final rematch with Latvian Anastasija Sevastova.
Sevastova toppled seventh-seeded Elina Svitolina 6-3, 1-6, 6-0, leaving just two of the top 10 women's seeds remaining.
Stephens, runner-up to Halep at the French Open, was looking forward to taking on Sevastova again in her bid to push ahead in her title defense.
"Obviously it won't be easy. Quarter-finals of a Grand Slam is always really tough. A big opportunity for both of us. Being defending champion, being able to get to the quarter-finals again, is incredible. I'm just going to try to keep building on that and keep going," she said.
Sevastova, in the quarters for a third straight year, is hoping to turn the tables on Stephens, who is trying to become the first woman to win back-to-back US Open titles since Williams won three straight in 2012, 2013 and 2014.
"Maybe third time lucky for me," Sevastova said.
Read Exclusive COVID-19 Coronavirus News updates, at MyNation.