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But she ended it screaming in delight as she finally suppressed Giulia Gatto-Monticone having been dragged into an unlikely dogfight by the world number 161.
The Italian qualifier, who was making only her second Grand Slam appearance at the age of 31 having amassed a relatively modest $US300,000 in prize money throughout a pedestrian career, was eventually subdued 6-2 7-5.
But she put the seven-times champion through her paces, testing the injuries that have limited the American to seven matches since the Australian Open in January.
Williams was rusty by her own admission, but a series of wayward shots that allowed the Italian to profit despite her obvious limitations did not dampen the American's delight at simply finishing the encounter feeling fit and healthy.
The knee injury that has plagued her, and contributed to a third-round exit at Roland Garros, did not rear its head, giving Williams hope that her form would shortly return too.
Yet she needs matches and fast - to the extent that she is considering playing mixed doubles with Britain's twice Wimbledon champion Andy Murray in an effort to get back to her best.
"I think right now I have to have every match count, like 10 matches, because I haven't had a ton of matches this year," Williams told reporters.
Gatto-Monticone came from the other side of the tennis tracks to the American, having only played one previous match against a top-20 player and never beaten someone ranked higher than 102.
Yet her staying power against the 23-times Grand Slam singles champion, who has amassed nearly $89 million in prize money, gave the fans on Centre Court a contest they could not have expected.
Gatto-Monticone, whose career is remarkable only for the late age at which she finally made a breakthrough, seemingly enjoyed her moment in the limelight.
Her story, said Williams, is a "great tale" of the value of persistence.
"It's nice because no matter where you are, you don't want to give up... Doesn't matter what age you are, whether you're 15 or in your early 30s, you still have a chance to be great at whatever you do," the world number 10 said.
"She really took it to me today. She's had some really good, strong matches in the past few months. Honestly, it's a good thing to see."
It did not look like the Italian had much fight in her when Williams took the opening five games but from then on, the pair went toe-to-toe, albeit in a scrappy encounter low on quality.
After taking the first set in 29 minutes, Williams had to wait until the eighth game of the second set to break serve only for the resilient Italian to fire straight back and then level at 5-5.
After getting her nose in front again, Williams brought up match point on Gatto-Monticone's serve, pumping her fists furiously before wrapping up the encounter following a lengthy rally and rapid exchange at the net.
Like an ageing heavyweight, the 37-year-old Williams clearly still packs a punch but looked well below par and desperately short of sharpness.
Next up for the former world number one will be a second-round clash against Kaja Juvan, a highly-rated 18-year-old prospect from Slovenia who showed off her potential with a 6-4 2-6 6-4 win over Kristyna Pliskova.