Miami, FL (SportsNetwork.com) - Serena Williams won her rain-delayed opener at the $5.3 million Miami Open in straight sets on Saturday.

Williams, who has won seven Miami titles, is back in competition after pulling out of her semifinal at Indian Wells last week because of a knee injury.

She had to wait an extra day to face Monica Niculescu after persistent rain washed out Friday's night session at Crandon Park. She aced Niculescu seven times in a 6-3, 6-1 win that took 81 minutes.

Williams earned a third-round match against promising 15-year-old American CiCi Bellis, who upset 29th-seeded Zarina Diyas 6-2, 6-1 on Friday.

Her sister, three-time Miami champion Venus, won her third-round match on Saturday. The 16th-seeded elder Williams topped No. 23 seed Samantha Stosur 6-4, 7-6 (7-3).

No. 4 seed Caroline Wozniacki was a third-round winner, fighting back from a set down to beat Kaia Kanepi 4-6, 6-1, 6-3 on the Crandon Park hardcourts. She will play Venus Williams in the next round.

Also moving on with third-round wins were No. 7 seed Agnieszka Radwanska, No. 8 seed Ekaterina Makarova, No. 9 seed Andrea Petkovic, No. 12 seed Carla Suarez Navarro, No. 14 seed Karolina Pliskova and Daria Gavrilova.

There were three other second-round matches pushed to Saturday because of the rain.

An upset came when qualifier Tatjana Maria topped sixth-seeded Canadian Genie Bouchard 6-0, 7-6 (7-4). No. 11 seed Sara Errani beat Anastasia Pavlyuchenkova 6-1, 7-6 (7-5) and 13th-seeded Angelique Kerber topped Heather Watson, 7-5, 3-6, 6-4.

The 2015 Miami champ will earn $900,400.