Seventh seed Venus, who beat Serena in the Wimbledon final this year, faces Poland’s ninth seed Agnieszka Radwanksa and fourth seed Serena plays unseeded Severine Bremond of France.
"I have had a great summer, winning Wimbledon and then the gold medal (in doubles) at the Olympics," she said.
"I have played a lot of matches…I am definitely at my best now."
Serena said she still had some way to go before hitting top form.
"I’m glad (Venus) feels like she’s playing her best. It’s nice to feel like when you’re playing your best. Right now I’m not playing my best, but that means I can do so much better."
Russian sixth seed Dinara Safina, runner-up at the French Open and Olympics, faces German qualifier Anna-Lena Groenefeld, while former world number one Amelie Mauresmo meets Italian 16th seed Flavia Pennetta.
In the men’s draw, top seed Rafael Nadal continues his quest for a first U.S. Open title when he plays American Sam Querrey in the third round.
Sixth seed Andy Murray takes on Swiss 10th seed Stanislas Wawrinka for a place in the last 16.
Kei Nishikori, the first Japanese man to reach the last 16 at Flushing Meadows, faces Argentine Juan Martin Del Potro, who has won his past 22 matches. Reuters