Venus, 32, seeking a record fourth Olympic Tennis gold, progressed with a 6-1 6-3 win over Canada's Aleksandra Wozniak.
Belarusian top seed Victoria Azarenka beat Spain's Maria Jose Martinez Sanchez 6-1 6-2 to also reach the third round, while Germany's Sabine Lisicki defeated Yaroslava Shvedova of Kazakhstan 4-6 6-3 7-5.
Russians Maria Sharapova and Maria Kirilenko combined to make it a frustrating day for Great Britain's women.
Third seed Sharapova struggled in the first set as Laura Robson forced a tie-break, but won out 7-6 (7/5) 6-3.
Kirilenko defeated Heather Watson in straight sets, 6-3 6-2.
Kirilenko said: 'I think it was a tough match. Every game I think we played great, all the rallies were amazing. The quality of tennis was good.'
Germany's Julia Goerges followed up her opening-round win over Wimbledon runner-up Agnieszka Radwanska by posting a 6-3 7-5 win against USA's Varvara Lepchenko.