Looking in good shape and highly mobile, she eased past Sweden's Rebecca Peterson 6-4, 6-2 to make the second round of the Italian Open in Rome.
Petersen looked set to push the former World No 1 going 3-1 up in the first set, but 10th-seeded Williams won five of the next six games.
Despite her lack of practice, tenth-seeded Williams was never threatened by Peterson, despite 22 unforced errors by the American in the first set.
Serena was playing for the first time since withdrawing ahead of her third-round match in Miami because of a left knee injury.
Up next for Williams will be the victor of another blockbuster first round between Venus - who won the title in Rome back in 1999 - and Qatar Total Open champion Elise Mertens.
She will play either her older sister Venus, who received a wild card entry to the tournament she won in 1999, or Belgian Elise Mertens in the second round. Serena leads 18-12 however Venus won her only meeting on clay in Rome 21 years ago.
"It was really cool to share my favourite city with her, some of my favourite things to see".
"It was a very long match", Venus said during her press conference. "I couldn't really tell when or how it was going to end, was happy it did". "I did everything I could to stay fit and keep my cardio up".
"Now when I go to tournaments, I really kind of want to get there early so I can have these different experiences and photos and videos and memories with Olympia".
"I love the clay season and wanted to do everything to be a part of it and I'm here".
In other action, 13th-seeded Borna Ćorić rallied past Felix Auger-Aliassime 6-7 (4), 6-3, 6-4 in a clash of two of the top young players on tour. Edmund took the first set 6-4, but Spaniard Verdasco hit back to win the second and third 6-4, 6-2 and wrapped up victory in two hours and 23 minutes.
Also, Karen Khachanov overcame a partisan crowd to beat Italian wild card Lorenzo Sonego 6-3, 6-7 (1), 6-3; while last year's French Open semifinalist Marco Cecchinato was inspired by the home fans to beat Alex Di Minaur 4-6, 6-3, 6-1.
Other ladies advancing included Madison Keys, Garbine Muguruza, Johanna Konta, Annett Kontaveit, Victoria Azarenka, Lucky Loser Amanda Anisimova, Sofia Kenin, Viktoria Kuzmova, Dominika Cibulkova and Katerina Siniakova.