"I feel good and just so happy to be back here". Williams, who identifies as "the most girlie out of all [five of her] sisters", also compared her body and how she's seen to her older sister and doubles partner, Venus Williams, who is leaner and taller than her.
Third-seeded Marin Cilic also started his Paris campaign with a straight-set win over James Duckworth, while up-and-coming Canadian teenager Denis Shapovalov got past John Millman in his Grand Slam debut 7-5, 6-4, 6-2.
After her opening match, Williams played down her chances of winning a joint-record 24th Slam, saying she was not putting any pressure on herself to equal Margaret Court's tally so soon into her comeback from a hard birth.
"That's a tough one for Serena; I honestly wouldn't expect her to win that", said Pat Cash, the former Wimbledon champion who is coaching the American player Coco Vandeweghe.
After falling behind 3-0 in the first-set tiebreaker, Williams reeled off six points in a row.
As Williams - and her eye-catching outfit - grabbed the headlines, there were wins but a few scares for 10-time champion Rafael Nadal and two-time victor Maria Sharapova.
Alas, the 22-year-old fell short in her quest to join fellow Australians Samantha Stosur and Daria Gavrilova in the last 32.
"The biggest difference is definitely I'm semi on time today", she continued.
She won her opening match against Krystina Pliskova, but struggled from the outset against Barty in the evening encounter on Philippe Chatrier.
Barty managed to win 73 percent of her total service points and had just 17 unforced errors.
"I don't even know what material it is, it looks like neoprene".
The world No.25 looked like suffering her third consecutive first-round defeat at Roland Garros before turning the match around after a 45-minute rain delay. "The connection that it gives me to Olympia, it's been one of the most magical things I've ever experienced".
A three-time champion at Roland Garros, Williams, 36, had not played a competitive match on clay this season ahead of her first-round match against Pliskova, who had 15 aces.
With the headlines dominated by the returns of Williams and Sharapova, world number one Simona Halep will begin her latest bid to win a maiden Slam very much under the radar.