Williams will be playing in her first major tournament since winning last year's Australian Open, having taken time off to give birth to her first child.
Halep, a two-time runner-up in Paris, knows that despite a lack of time on court, the unseeded Williams could prove a threat.
"It's great for tennis that she's back," said Halep ahead of the start of the tournament on Sunday.
"Maybe she needs a little bit more time to get again used to the tournaments and everyday playing.
"In my opinion... She's able to come back and to win tournaments again.
"She knows how to handle the pressure, how to handle all the situations. She's (been) there for so many years. So I think she will come back and she will be good again."
Williams, who opens her campaign with a first-round match against Czech Kristyna Pliskova, struggled on her return to tennis in March with early defeats in Indian Wells and Miami before taking more time off to get fully fit for Roland Garros.
The 23-time Grand Slam champion is joined in Paris by fellow former world number ones Victoria Azarenka and Maria Sharapova, with the trio playing a major tournament together for the first time since the 2016 Australian Open.
"I feel that now everyone is here (it) feels that the tournament is completed and everyone is here just to show that tennis is really nice and also that everyone can win it," added Halep.
"These girls are coming back, Vika (Azarenka), Serena, with the kids, which is amazing. Makes it a little bit different and also special."
Romanian Halep will be looking to end her wait for a first Grand Slam title after three previous final defeats, including last year at Roland Garros by Jelena Ostapenko and in February's Australian Open against Caroline Wozniacki.
"It's a Grand Slam. Everyone is at the highest level here," she said.
"So it's going to be difficult. I'm not thinking about the title, because it's really far. I'm thinking just of my first match."
The top seed will take on American world number 105 Alison Riske in the first round.