US Open, Flushing Meadows
Dates: 30 August - 12 September Start time: 1600 BST
Coverage: Live text commentary on BBC Sport website and mobiles; updates on BBC Radio 5 live; live on Sky Sports
Williams is playing her first tournament since Wimbledon
Third seed Venus Williams was given a stern test by Canadian qualifier Rebecca Marino before sealing a place in round three of the US Open.
The two-time champion eventually came through 7-6 (7-3) 6-3 on a swelteringly hot day in New York and will play Mandy Minella of Luxembourg next.
Former world number one Ana Ivanovic continued to impress as she blew away 21st seed Jie Zheng of China 6-3 6-0.
Defending champion Kim Clijsters beat Sally Peers 6-2 6-1 in the night match.
Despite the scoreline it was an uneven display from the Belgian, who got just 51% of her first serves in and committed 21 unforced errors.
"I just had to find my footing again a little bit," said Clijsters. "I felt as the match went on I got to read her game a bit better and moved a bit better, but I still have a little further to go before I play my best tennis."
Tenth seed Victoria Azarenka was taken to hospital after collapsing on court during her match with Gisela Dulko and later revealed she had suffered mild concussion following an earlier fall in training.
It was challenging, not just with the conditions, but also my opponent
Dulko will now play rising Russian star Anastasia Pavlyuchenkova after the 20th seed ended the hopes of Sania Mirza of India 6-2 6-4.
Ivanovic, who is now ranked 40 in the world and working with Steffi Graf's former coach Heinz Gunthardt, was hugely encouraged as she cruised into the third round.
The Serb has slipped from being number one in the world two years ago after injuries and a dramatic loss of confidence, but appears to be on her way back now.
"I remember a couple years ago when I was here and I was saying, even though I'm number one I don't feel I'm playing as number one," said Ivanovic.
"But now I feel like I'm playing like a top-10 player, I have confidence that I can beat these players. That's huge for me.
"I feel as if I am just coming up again, and I have really nothing to lose. I've got that joy of competing again."
Williams admitted that qualifier Marino had pushed her hard, saying: "It was challenging, not just with the conditions, but also my opponent.
"She served so well and mixes up her shots. It seemed like every time I had an opening she came up with a big serve, so I guess I know what it's like now playing myself."
Ivanovic goes on to face Virginie Razzano, who ousted 13th seed Marion Bartoli 7-5 6-4 as her fellow Frenchman was hampered throughout by a thigh injury that required heavy strapping.
French Open champion Francesca Schiavone hammered Maria Elena Camerin 6-2 6-1, while 24th seed Daniela Hantuchova fought back to beat American Vania King 5-7 6-3 6-4.
Number 12 seed Elena Dementieva had little trouble seeing off Austria's Sybille Bammer 6-3 6-4, and fifth seed Samantha Stosur beat fellow Australian Anastasia Rodionova 6-1 6-4.