MIAMI, March 21 (Reuters) - Serena Williams, playing her first match since regaining the world number one ranking, made a triumphant start to the Sony Open, thumping Italy's Flavia Pennetta 6-1 6-1 on Thursday.
Playing her first match after a bye in the opening round, Williams made a sloppy start, hitting two double faults in her opening service game, but was unstoppable once she found her rhythm.
"I think I was off, footwork-wise, but it's always good to win a match and then just keep going," Williams said.
"There's nothing like when they announce you and they say, 'the number one player in the world.'
"It definitely gives me a little bit more confidence, I think a little bit more pep in your step. It's just a great feeling."
At 31, Williams is already the oldest woman to reach the top of the rankings but says she feels as young as ever and has lost none of her motivation as she chases a sixth Sony Open title.
"Growing up this was dubbed the fifth grand slam - it's not a grand slam, but it's definitely a great tournament," Williams said.
"It would be really cool to have the record. But I have a lot of matches to play these two weeks."
China's Li Na also made a flying start to the tournament, winning her first match since losing in the Australian Open final.
Li, beaten by Victoria Azarenka in Melbourne, reeled off the last 10 points of the match to beat Kiki Bertens of the Netherlands 6-3 6-1.
"My game was pretty good today," said Li. "I followed the plan of what I should do."
Defending champion Agnieszka Radwanska fired down six aces in a 6-3 6-2 win over Taiwan's Hsieh Su-Wei.
"I will do everything to hold the trophy again," said the Pole. "Winning this tournament is almost like winning a grand slam."
Former Wimbledon champion Petra Kvitova and ex-world number one Caroline Wozniacki also won, but both had to fight all the way.
Kvitova rallied from a set down to beat Peng Shuai 5-7 6-2 6-2 while Wozniacki did the same to beat Czech qualifier Karolina Pliskova 5-7 6-3 6-3. (Reporting by Julian Linden in New York; Editing by Ian Ransom)