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."
Williams's sister Venus, seeded 19 in Miami, saw off Japanese veteran Kimiko Date-Krumm 7-6(3) 3-6 6-4 to seal her progress into the third round in a late-finishing match.
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.
WTA Miami second-round results
19-Venus Williams (U.S.) beat Kimiko Date-Krumm (Japan) 7-6(3) 3-6 6-4
25-Varvara Lepchenko (U.S.) beat Irina Begu (Romania) 6-3 6-4
Romina Oprandi (Switzerland) beat 17-Lucie Safarova (Czech Republic) 7-6(4) 4-6 7-6(5)
Andrea Petkovic (Germany) beat 10-Marion Bartoli (France) 6-3 4-1 (Bartoli retired)
Magdalena Rybarikova (Slovakia) beat 27-Mona Barthel (Germany) 6-3 7-6(5)
9-Caroline Wozniacki (Denmark) beat Karolina Pliskova (Czech Republic) 5-7 6-3 6-3
Garbine Muguruza Blanco (Spain) beat 23-Anastasia Pavlyuchenkova (Russia) 4-6 6-2 6-2
13-Dominika Cibulkova (Slovakia) beat Kristina Mladenovic (France) 6-2 6-3
Ayumi Morita (Japan) beat 31-Yanina Wickmayer (Belgium) 7-6(2) 2-6 6-3
1-Serena Williams (U.S.) beat Flavia Pennetta (Italy) 6-1 6-1
7-Petra Kvitova (Czech Republic) beat Peng Shuai (China) 5-7 6-2 6-2
16-Sloane Stephens (U.S.) beat Olga Govortsova (Belarus) 0-6 6-4 6-4
30-Kirsten Flipkens (Belgium) beat Stefanie Voegele (Switzerland) 6-4 6-2
Ajla Tomljanovic (Croatia) beat 24-Julia Goerges (Germany) 7-6(4) 6-2
5-Li Na (China) beat Kiki Bertens (Netherlands) 6-3 6-1
4-Agnieszka Radwanska (Poland) beat Hsieh Su-Wei (Taiwan) 6-3 6-2
ATP Miami first-round results
Thomaz Bellucci (Brazil) beat Daniel Brands (Germany) 2-6 6-4 6-2
Aljaz Bedene (Slovenia) beat Benjamin Becker (Germany) 6-4 6-3
Simone Bolelli (Italy) beat Jesse Levine (Canada) 4-6 6-3 7-6(4)
Jarkko Nieminen (Finland) beat David Nalbandian (Argentina) 2-6 6-4 6-3
Guido Pella (Argentina) beat Carlos Berlocq (Argentina) 2-1 (Berlocq retired)
Lukasz Kubot (Poland) beat Frank Dancevic (Canada) 4-6 6-4 6-3
David Goffin (Belgium) beat Robin Haase (Netherlands) 7-6(4) 3-6 6-1
Viktor Troicki (Serbia) beat Pablo Andujar (Spain) 6-2 6-3
Bernard Tomic (Australia) beat Marc Gicquel (France) 7-5 7-6(3)
Santiago Giraldo (Colombia) beat Rhyne Williams (U.S.) 6-2 6-4
Guillaume Rufin (France) beat Marius Copil (Romania) 6-4 4-6 6-1
Olivier Rochus (Belgium) beat Tatsuma Ito (Japan) 4-6 6-4 3-1 (T. Ito retired)
Alejandro Falla (Colombia) beat Go Soeda (Japan) 7-5 6-1
Lu Yen-Hsun (Taiwan) beat Robby Ginepri (U.S.) 7-5 6-1
Daniel Gimeno-Traver (Spain) beat Denis Istomin (Uzbekistan) 6-2 6-4
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