Matches at last year's tournament in September stretched into the night on courts with new lights. But scheduled night play is new this year at Roland Garros.
Williams and Begu took the court in the twilight at 9 p.m. local time with the stadium stands empty. Fans are being admitted for day matches this year, but not for night sessions.
"It's definitely a little different, but overall, ironically enough night sessions are not my favorite matches, but I do have a good record at it so clearly something about it gets me hype," Williams said. "I have to say it was really cool to be able to play the first night session ever, here at Roland Garros, so that was something that I thoroughly enjoyed."
Williams struggled with her first serve but moved well and played aggressively, charging forward when she had the chance. Facing a set point in the tiebreaker, she sprinted in and, from the service line, whacked a nervy swinging volley for a winner.
She has won the French Open three times, most recently in 2015, and at age 39 is chasing a record-tying 24th Grand Slam singles title. She improved to 77-1 in first-round Slam matches. The loss came in Paris in 2012.
Earlier Monday, defending French Open champion Iga Swiatek picked up right where she left off last year in winning the title: running yet another opponent this way and that on the red clay of Roland Garros in a straight-sets victory.
Her best friend on the tennis circuit, Kaja Juvan, was the victim on Court Philippe Chatrier this time as Swiatek emphatically kicked off her campaign to become the first woman since Justine Henin in 2007 to defend the French Open title.
Playing on her 20th birthday, Swiatek treated herself to a 6-0, 7-5 victory -- her eighth straight-sets win in a row at Roland Garros, having also not dropped a set in winning the trophy last year as an unseeded 19-year-old.
Another former Grand Slam winner didn't fare as well as Swiatek on Monday. Bianca Andreescu, the 2019 US Open champion, is out in the first round after losing a 9-7 third set.
The sixth-seeded Andreescu's second appearance in the main draw at Roland Garros ended with a 6-7 (1), 7-6 (2), 9-7 defeat against 85th-ranked Tamara Zidansek of Slovenia. Zidansek had previously been 0-2 at the French Open.
Andreescu withdrew before her quarterfinal last week in Strasbourg with an abdominal injury.
Muguruza received treatment for a back issue between sets and made a slew of aggressive errors, as though eager to end rallies quickly.
She won the tournament in 2016, won Wimbledon in 2017 and was seeded 12th. It was her first opening-round exit at the French Open.
Kostyuk, ranked 81st, earned her first win over a top 20 player.
Kenin is seeded fourth and was the runner-up last year to Swiatek. The victory was Kenin's first since announcing she's no longer coached by her father, Alex.
Kenin converted nine of 10 break points against Ostapenko and won despite losing serve seven times.
The Associated Press contributed to this report.