Competing in her first Grand Slam since winning the 2017 Australian Open while pregnant, and in one of few tournaments since delivering her daughter last September, Williams defeated Kristyna Pliskova, 7-6 (7-4), 6-4, in her first-round match at Roland Garros on Tuesday.
Her return to tennis came earlier this year but she entered the French Open on a two-match losing streak and with no clay matches under her belt.
The temperature hit 25C in Paris but it did not seem to affect the 23-time major victor in her first Grand Slam match since giving birth last September.
Nadal had to save four set points in the tiebreaker before wrapping up a victory.
Williams then trailed 0/3 before eventually taking the set when Pliskova, who had never won a match at the tournament in three previous visits, put a forehand wide.
Jordan Thompson lost to Casper Ruud in a five-set thriller, young-gun Alex de Minaur went down to Britain's Kyle Edmund in three sets, and James Duckworth was convincingly beaten by no. 3 seed Marin Cilic.
Pliskova, a lefty whose twin sister upset Williams in the 2016 US Open semifinals, actually hit more aces, 15. Muguruza, who claimed the title in Paris two years ago, advanced to the second round with a 7-6 (7-0), 6-2 win.
As French organisers rigidly use WTA rankings to determine their seedings she is the most unsafe of floaters. "In the beginning it was hard to learn her nap schedules... because I don't want her to ever feel like I'm not around".
But, like the great champion that she is, Williams was not going to settle...
"If I didn't miss it, I wouldn't be out here", Williams said.
"It was really exciting to see so much African-American culture impacted in the wedding", she said.
Serena Williams has always played like a superhuman. Later on, Pliskova's efforts to stretch Williams out on longer rallies didn't always pay off. Williams "played better than I expected", Pliskova said bluntly.
"I'm really fortunate enough that I can plan my days and I can plan my career", said Williams, who appeared to symbolise her life as a "Super-mom" in the skin-tight, black catsuit she chose to wear for Tuesday's match. But there was no slip in her game as she closed it on her second match point. "I think there is a donation to charity for each ace, so both of us helped".
"I battled the rain, my opponent, the nerves, so it's pretty nice to get through".