Former world number one Williams returned to the WTA Tour last March, just six months after giving birth to her daughter and suffering complications in the aftermath, and managed to reach the finals of Wimbledon and the US Open.
The American, who was forced to pull out of the French Open due to a shoulder issue, lost to Angelique Kerber at the All England Club and went down to Naomi Osaka at Flushing Meadows, a match that was overshadowed by her dispute with umpire Carlos Ramos.
Williams' hopes of drawing level with Margaret Court on 24 grand slam wins at this year's Australian Open were extinguished by Karolina Pliskova at the quarter-final stage, but Mouratoglou assured her ambitions remain high for the rest of 2019.
"Her target has always been to win grand slams and that's why she came back, because she believes – and of course I believe too – that she is able," Mouratoglou said at the Laureus World Sports Awards 2019.
"She has the level to win grand slams. She was close two times last year, played to grand slam finals, so this year the goal is to do better than last year and win the last two that are missing, and then we see.
"What Serena has achieved over the last 20 years is unique in the history of the sport, but she's a unique person, she's a unique player and we will do everything to beat that record, so she stays in all the [history] books forever."
Mouratoglou was impressed that Osaka managed to follow up her maiden major triumph at the US Open by winning the Australian Open, though the world number one's decision to split with coach Sascha Bajin surprised him.
"Naomi achieved something incredible. She won her first grand slam and right after she won another, which is something so rare in our sport, whether for men or women," he said.
"Usually you need a lot of time to recover and you experience a new pressure as a grand slam winner and it takes time and she's been able to bounce back and this shows incredible mental strength.
"It's clearly strange [the split with Bajin], it's not something you're used to seeing. It's her choice and we have to respect it and she probably had her reasons and they didn't want to talk about it, so we have to respect it."