Serena Williams continued her march in the U.S. Open by easing past Maria Jose Martinez Sanchez 6-2, 6-4, but her elder sister Venus fell to in-form German Angelique Kerber in a tough encounter 6-2, 5-7, 7-5.
Wimbledon finalist Agnieszka Radwanska suffered a first-set scare, before fighting back to beat Carla Suarez Navarro 4-6, 6-3, 6-0.
Serena was not at her best in the second round match, making 24 unforced errors, but her experience shone through in the end, making the scoreline look easier than it actually was.
"She always plays me tough," Serena said after the match.
The 30-year-old American will now play Ekaterina Makarova, another left-handed player, in the third round. Makarova has already got the better of Serena in a Grand Slam this year, beating the American 6-2, 6-3 in the Australian Open fourth round, a loss that Serena is extremely aware of.
"It was a good match for me today because I played a lefty," Serena said. "I look forward to it -- she jumpstarted my year career this year."
After the loss, Serena went on to lose in the French Open first round, but bounced back brilliantly by taking the Wimbledon and Olympic titles on the grass-courts of the All-England club.
Serena said she was disappointed to see Andy Roddick announce his retirement, but admitted is came as no surprise. "He told me a while ago, last year, that this would be it," she said. "He told me again when I was at his house in Austin at the end of the year.
"I was thinking, 'Please change your mind.' It's tough when you don't want to go out there and do the work to get ready and the preparation.
"Ever since I have been on tour, it feels like Andy has been there. He has been great for American tennis, great for the U.S. Open, doing so much, playing so well, just being such a great player.
"He has a great attitude and a lot of people look up to him. It's incredibly sad for me to lose a friend on tour. It's going to be hard."
Serena has no plans of retiring anytime soon though. "I'm not retiring, just for the record," she added. "You haven't seen the last of me yet. It has been a crazy ride, really exciting. I don't want it to end, so I'm going to continue."
Serena's sister Venus was disappointed to bow out of the singles draw, after her loss to Kerber in the night session. "Losing a match like this isn't a lot of fun, but that's sport," Williams said. "All I had was fight because I didn't play well.
"I was trying hard but I was making errors everywhere. I made 60 errors. It's hard to win a match like that. I was fighting her and me."