Serena Williams moved into a career tie in US Open wins with her sister, Venus, after a 6-4, 6-0 victory over Ekaterina Makarova at Flushing Meadows.
Tennis: Agnieszka Radwanska and Serena Williams move on
The Olympic and Wimbledon champion won eight games in succession to claim a 6-4, 6-0 win over 42nd-ranked Ekaterina Makarova.
Williams, the fourth seed, fired 31 winners and eight aces to triumph and she acknowledged she had to work hard to succeed.
"After 4-4, I started to play better and more consistent. She's a good opponent, she beat me in Australia so I knew I needed to play better and I did," the 1999, 2002 and 2008 winner said.
Williams now has a 61-9 career record in New York, to match her older sibling, who was knocked out in the second round on Friday by Angelique Kerber.
"I can't leave you guys," she joked to reporters during her post-match news conference. "I love this sport way too much. I need to stay here for tennis."
Agnieszka Radwanska, the second-seeded Pole, was relieved to beat Jelena Jankovic in straight sets 6-3, 7-5 to reach the fourth round for the first time at the US Open in her career.
"I was feeling like I was melting there," said Radwanska, who played with heavy strapping on her right shoulder. "I survived the match. In the second set, I was just better for a couple of points ...
"The shoulder is quite good, no complaints. I have played three very good matches here."
Her next opponent is Roberta Vinci, who beat No 13 seed Dominika Cibulkova 6-2, 7-5 to equal her best grand slam performance.
In men's action, Spanish 11th seed Nicolas Almagro and 12th seeded Croatian Marin Cilic made the last 16.
Almagro reached the fourth round at the US Open for the first time with a 7-6, 6-7, 7-6, 6-1 win over American wild-card Jack Sock.
Cilic, a quarter-finalist in 2009, ended Asian hopes in the singles when he beat Japanese 17th seed Kei Nishikori 6-3, 6-4, 6-7, 6-3.
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