Miami, FL (SportsNetwork.com) - Serena Williams' opening match at the $5.3 million Miami Open was rained out Friday night.

The top-ranked Williams was scheduled to face Monica Niculescu but persistent rain washed out the night session, forcing organizers to postpone a handful of matches.

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

The winner of her match will face promising 15-year-old American CiCi Bellis, who upset 29th-seeded Zarina Diyas 6-2, 6-1 on Friday.

Third-seeded Simona Halep, fresh off her big title at Indian Wells, moved on with a 6-4, 2-6, 6-1 win over Nicole Vaidisova on Crandon Park hardcourts. Vaidisova, 25, returned to the WTA this year after retiring from the sport back in 2010.

Halep has won 11 consecutive matches and is 20-1 so far this season, her lone setback coming in the quarterfinals of the Australian Open in January. She's seeking her third title in her last three events.

In a battle of former world No. 1s, two-time Miami champion Victoria Azarenka cruised to a 6-1, 6-1 win over 20th-seeded Jelena Jankovic, who was fresh off a runner-up finish to Halep last week in the California desert.

A top-10 upset came when Johanna Larsson beat No. 10 seed Lucie Safarova 7-6 (7-5), 6-2, while 15th-seeded Flavia Pennetta moved on by overcoming Tsvetana Pironkova 6-7 (1-7), 6-3, 6-3.

Meanwhile, Sloane Stephens upended 17th-seeded fellow American Madison Keys 6-4, 6-2.

Also winning on Day 4 were Garbine Muguruza; 2006 Miami titlist Svetlana Kuznetsova, who beat American Alison Riske in straight sets; Sabine Lisicki and Camila Giorgi whipped Belgian Alison Van Uytvanck 6-3, 6-2.

The 2015 Miami champ will earn $900,400.