World number one Halep fought back after a disastrous opening set to beat American Alison Riske 2-6, 6-1, 6-1 in their delayed first-round match.
Halep, the runner-up in Paris in 2014 and 2017, slumped 0-5 down in the opening set to world number 83 Riske whose only win at the tournament came in 2014.
The Romanian committed 16 unforced errors in the set but stepped up a gear from that point on to book a second-round match against Taylor Townsend of the United States.
"It's always difficult in the first round of a Grand Slam, you are always nervous," said 26-year-old Halep after a match that was pushed back from Tuesday due to rain.
Halep needs to reach the semi-finals or better if she is to retain the world top spot.
Her main rival for the top ranking, second seed Caroline Wozniacki, joined her in the last 32 by thrashing Spanish qualifier Georgina Garcia Perez 6-1, 6-0 in only 51 minutes.
Eighth seed Petra Kvitova eased into the third round with a 6-0, 6-4 victory over Spain's Lara Arruabarrena on Court One.
The two-time Wimbledon champion has now won 13 consecutive matches on clay after claiming titles in Prague and Madrid, and will face Estonia's 25th seed Anett Kontaveit for a place in the last 16.
Ukrainian fourth seed Elina Svitolina reached the last 32 for the fourth straight year by brushing aside Viktoria Kuzmova in straight sets.
The 23-year-old, who arrived in Paris having won her second straight Italian Open title, saw off her Slovakian opponent 6-3, 6-4 and will next take on Romanian 31st seed Mihaela Buzarnescu.
Elsewhere, charismatic Japanese 21st seed Naomi Osaka came through a tight match with Kazakhstan's Zarina Diyas 6-4, 7-5.
Serena Williams, who made her Grand Slam return with a singles win on Tuesday, partnered sister Venus in a 4-6, 6-4, 6-1 doubles victory over Japan's Shuko Aoyama and Miyu Kato.
