Williams, who donned a fabulous all-black catsuit for her return to the Grand Slam stage, said the form-fitting choice was a shout out to "all the moms out there that had a tough pregnancy and had to come back and try to be fierce".
The 23-time Grand Slam champion is playing in her first major championship since the 2017 Australian Open.
Williams had played just two previous WTA tournaments since giving birth to her daughter, Alexis Olympia.
The 23-time major victor, who had not played on clay since 2016 after taking time away to have a baby, likened her outfit to something from Marvel's Black Panther and said the new look made her "feel like a super hero".
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The last time she had been spotted on a court was in a first-round loss to Naomi Osaka in Miami in March. On Sunday, Venus lost her first-round match to China's Wang Qiang, but several American women advanced, including 2017 US Open champ Sloane Stephens, who defeated Arantxa Rus, and CoCo Vandeweghe, who beat Laura Siegemund.
An increasingly unpredictable set saw Williams break again, the third break in a row and fifth in the set overall.
- Despite enormity of occasion, the 36-year-old, with her hair scraped back into severe-looking bob, the stadium is barely half-full as fans take a breather following Rafael Nadal's opening win. Both the Czech world number 70 and Williams avoided errors early on and it seemed inevitable that the set would be decided on a tie-break.
Serena Williams has stepped out in a skin-tight outfit at the French Open. "But I trained really hard on the clay, and I feel good, and I'm just happy to have won a match here".
Dressed in a skintight, all-in-one black bodysuit, broken up by a vivid scarlet belt, Serena captivated the French crowd from the start of the contest to its 7-6 (4) 6-4 conclusion.
Garcia, one of 10 French women in the main draw, the second most of any country after US with 19, gave Duan her 13th defeat in 13 meetings against Top 10 players.