World number one Ashleigh Barty and defending champion Angelique Kerber also made it comfortably through.

Williams, 37, won 6-2 7-5 in one hour and 19 minutes on Centre Court.

French Open champion Barty overcame Chinese world number 43 Zheng Saisai 6-4 6-2 and will now play Belgian Alison van Uytvanck.

Kerber saw off fellow German Tatjana Maria 6-4 6-3 and faces American world number 95 Lauren Davis in round two.

Williams, the 11th seed, is looking to equal Australian Margaret Court's all-time record of 24 Grand Slam singles titles but has struggled with a knee injury this year.

The American had the chance to serve out the match at 5-3 in the second set only for 31-year-old Gatto-Monticone, making her debut in the Wimbledon main draw, to break back.

Williams roared in delight as she set up match point at 6-5 and then clinched the best exchange of the match with a volleyed winner to secure her spot in the next round.

"I really appreciate she never gave up and it's good for me to play matches like this," said Williams, after only her 15th singles match this year.

"It's the best I have felt since February. I had to skip most of the hard-court season, a couple of matches on clay, I'm feeling good that I can actually walk. It's just amazing.

"I am better now. I feel so thankful and I am happy, and now I get to play with my baby. It's fun to do everything I love."

Barty said it felt "incredible but bizarre" to be playing at Wimbledon as world number one.

The Australian was broken early in the first set against Zheng but otherwise seemed relaxed as she disposed of her opponent with a variety of slices and good approach play at the net.

The 23-year-old said: "The first round is always tough and it took a bit of time to get used to conditions on that beautiful court which I have a lot of happy memories on.

"It feels a bit bizarre actually but I'm trying to go about my business in the same way as I always have. You have to enjoy every minute when you're playing at this beautiful tournament."

World number five Kerber has been in good form on grass coming into Wimbledon having reached the semi-finals in Mallorca before reaching the final at Eastbourne where she was beaten by Karolina Pliskova.

The German said: "Coming back here, walking out on Centre Court is really special and there was a lot of emotion and memories. It's amazing to be back."