There were some difficult moments, though. Serving in the first game of the match Williams hit two double faults in the first three points and fell behind 0-40 - she also converted on just five of 18 break point chances in the match. But she also fended off all seven of Pennetta's break chances and after an hour and 17 minutes the No.1-ranked, No.1-seeded Williams was through to the next round.
"It was a little frustrating because I felt like I should be putting the balls away more quickly, but she was so quick and was running every ball down," Williams said. "I was trying to get myself more pumped up and more energetic out there, because I believe she was ranked in the Top 10 at one point.
"I think I was off, footwork-wise, but it's always good to win a match and then just keep going."
Williams was playing her first match since becoming the oldest player to rank No.1 in the world.
"My attitude is that it always feels better when you're No.1," Williams commented. "There's nothing like when they announce you and they say, 'The No.1 player in the world.' So for me 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."
With such a legendary career already, what could possibly keep Williams motivated to keep going?
"This keeps me going - I'm No.1, I'm pretty good at it still. If I'm doing something well, I don't want to stop, and I shouldn't stop. I don't have to play another match, but I'm having fun, I'm enjoying it.
"Normally the history books wouldn't really matter to me but when you get closer to a goal, you can't help but start thinking, 'Okay, I want to do history.' In retrospect I have made history already, so I just try to keep it going a little bit at a time. I don't know if I could ever top Margaret Court's record, but it would be exciting to try and reach some of my fellow countrywomen who are ahead of me."
This fortnight Williams could become the first player ever to win the Sony Open Tennis six times - she is currently tied for the all-time record with Steffi Graf, having won the Premier title five times.
"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. If I can just stay relaxed, I feel like I can do it. But sometimes I'm my own worst enemy. I just need to get through every match, one at a time, and we'll see what happens here."
Elsewhere, Petra Kvitova and Caroline Wozniacki were grinding out three set wins, with the No.7-seeded Kvitova rallying from a set down to beat Peng Shuai, 57 62 62, and the No.9-seeded Wozniacki doing the same to beat Czech qualifier Karolina Pliskova, 57 63 63. No.13 seed Dominika Cibulkova also moved through but only needed two sets to do it, out-hitting Kristina Mladenovic, 62 63.
A few of the lower seeds weren't so lucky, with No.23 seed Anastasia Pavlyuchenkova falling to Spanish wildcard Garbiñe Muguruza, 46 62 62; No.27 seed Mona Barthel missing out on two set points in the second set and falling to Magdalena Rybarikova in straight sets, 63 76(5); and No.31 seed Yanina Wickmayer succumbing to the two-handed blasts of Japanese No.1 Ayumi Morita, 76(2) 26 63.