The 37-year-old looked fitter and more mobile during a 6-4, 6-2 first-round win against Rebecca Peterson, a Swedish qualifier, at the Italian Open in Rome.
After a shaky start, Williams began to take control with her baseline power on a windy day at the Foro Italico.
Despite her lack of practice, tenth-seeded Williams was never threatened by Peterson, despite 22 unforced errors by the American in the first set.
Williams was playing for the first time since withdrawing ahead of her third-round match in Miami because of a left knee injury.
Two days on from Jannik Sinner's surprise win against Steve Johnson, the crowd in Rome was given further reason for cheer as Berrettini reached the last 16 of a Masters 1000 event for the first time.
She could yet meet Jo Konta, providing the younger sibling comes through one of those awkward matches against sister Venus that is her second round match-up.
Next up at the Italian Open: Sister Venus Williams in the siblings' first meeting on European clay in almost 17 years.
Serena is playing only her fourth tournament of the season and was unable to finish her last two due to physical ailments.
Said Serena, "I vaguely remember that, so I don't really remember", she said". In their last match in Rome, Venus won their second career meeting way back in the 1998 quarterfinals.
"It was a very long match", Venus said during her press conference.
Former world number four Dominika Cibulkova, won her match 6-2, 6-3 against Aliaksandra Sasnovich, and next meets US Open and Australian Open victor Naomi Osaka of Japan. 'It's been terrible. I've been working as hard as I could keeping up my cardio as much as I could with a knee injury'.
"It was really cool to share my favourite city with her, some of my favourite things to see. I knew that I love the clay season and I wanted to be a part of it".
The two friends go way back to their early days on tour as Amritraj is a former Top 200 player.
Other results included Anett Kontaveit, Dominika Cibulkova and Garbine Muguruza all advancing. Auger-Aliassime struggled with his serve, hitting seven double-faults to Coric's one.
Spanish veteran Fernando Verdasco also had to come from behind to beat Kyle Edmund 4-6, 6-4, 6-2.