Williams did not lose her serve against Rezai

The unseeded American, 26, broke the Frenchwoman twice in both sets and took just an hour to record the victory.

"It was the most comfortable I've felt this season," said Williams. "I'm rounding into form at the right time."

Meanwhile, Nadia Petrova made a comfortable start to her title defence with a 6-4 6-0 win over Kaia Kanepi.

Williams, who won her opening event of the year in Memphis in February, was near perfect on her first serve and out-foxed Rezai with her forehands.

"My service game was that strong in Memphis too, and my second serve was also very strong tonight," said Williams. "Yet last week at the Sony Ericsson in Miami I couldn't find my serve."

Petrova beat Francesca Schiavone in last year's final

Williams now faces number four seed Patty Schnyder, who has never beaten her in their eight meetings.

Petrova won the last eight games against Kanepi and will next meet Zheng Jie of China, who beat Italy's Mara Santangelo 6-3 6-3.

"The key was getting that first set under my belt and gaining some confidence," said Petrova.

"I played with a little more freedom in that second set. I had to adjust to sliding on the court. It's quite difficult to judge the distance if you start sliding before you hit the ball."

Fellow Russian and fifth seed Dinara Safina went through with a 6-4 7-5 win over Michaella Krajicek. Third seed Daniela Hantuchova beat Nuria Llagostera Vives 6-1 6-3.