Serena Williams into second round of Sony Open in Key Biscayne

Serena Williams is playing her first event since becoming the oldest No. 1 in WTA history.

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Playing again at No. 1, Serena Williams routed Flavia Pennetta of Italy 6-1, 6-1 on Thursday in the second round of the Sony Open.

Williams is playing her first tournament since she regained the No. 1 ranking at the Qatar Open. Williams lost the Doha final to Victoria Azarenka, but overtook her for the top ranking the week of Feb. 11.

Williams trailed 40-0 in the first and fifth games in the first set, but went on to hold serve both times. In the first game, she uncharacteristically double-faulted on the first and third points.

"I don't think it was too much rust,'' Williams said. "I think I missed a few shots, but only because I was off footwork-wise. It's always just good to win a match and stay in the tournament and keep going.''

Williams is trying to break the tournament record of five titles she shares with Steffi Graf.

"That would be really cool,'' Williams said. "I have a lot of matches to play. If I can just stay relaxed, I really feel like I can do it, but I'm sometimes my own worst enemy.''

Sloane Stephens rallied to beat Olga Govortsova of Belarus 0-6, 6-4, 6-4. Stephens is ranked 16th, the highest-ranked American after Williams.

Stephens won only four points in her three service games and 10 points overall in the opening set.

"I wasn't playing that great,'' Stephens said. "I was asleep out there. I definitely just needed to wake up and start playing. A lot just battling and just hanging in there.''

Stephens will play the winner of the match between 32-year-old Venus Williams and 42-year-old Kimiko Date-Krumm of Japan.

Poland's Agnieszka Radwanska, the defending champion who is seeded fourth, defeated Taiwan's Su-Wei Hsieh 6-3, 6-2 in the second round.

"Just very happy to have a quick match,'' Radwanska said. "The first match is always a little bit tough and tricky.''