Slovenia's Kaja Juvan, 18, appeared to have being inspired by fellow qualifier and teenager Cori Gauff's sensational first-round win over Williams's elder sister Venus, a five-times winner at the All England Club, as she came out swinging to break Serena twice in the opening set.
An ugly smash into the net not only handed Juvan the set but also left an angry Williams staring at the ball in disgust -- and if looks could kill, the ball would have turned to dust.
But if the Court One fans feared that it might be a case of "Gone on the Fourth of July" for America's most decorated tennis player, she stormed back for a 2-6 6-2 6-4 second-round win.
The 37-year-old missed easy slam-dunk smashes, she belted the ball wide while on break point and glared down on the net after yet another of her shots got tangled up at the bottom of the black mesh.
But despite producing more unforced errors (26) than winners (25), she put on a winning show for her friend Meghan Markle, the Duchess of Sussex, to seal a place in the third round against Germany's Julia Goerges by firing down her sixth ace.
"I play my best when I am down; it brings out the best in me. I'm just a fighter and I never give up," said a relieved Williams, who is chasing a record-equalling 24th Grand Slam title.
That was certainly true on Thursday because during the first hour of her contest with Juvan, things were not exactly looking rosy.
Williams had never lost to a player ranked outside the top 100 at the All England Club nor to a qualifier at any of the slams.
SCHOOL EXAMS
She had a 102-13 win-loss ratio on grass and a 338-47 record at the slams.
Taking on a Wimbledon debutante whose Grand Slam record stood at a slightly more modest 1-1, the result appeared to be a foregone conclusion.
However, Juvan, who will return to her homeland to finish her high school exams, appeared to have done her homework on her more illustrious rival as she belted winners galore to race to a 3-0 lead in the first set.
She barely put a foot wrong as she converted both of the break points she earned and made only two unforced errors while Williams failed to make a dent in the Slovenian's serve.
When the American finally broke Juvan in the opening game of the second set, it was game on.
Williams had a chance to grab a double break for a 4-1 lead but Juvan played like a seasoned professional as she refused to get intimidated by her rival's reputation.
The world number 133 saved break point by luring the 11th seed into the net with a dropshot and then just stood back to watch a screaming Williams wildly belt the ball into the backboards.
Williams did go on to win the set, and eventually the match, but she knows that if she wants to hoist the Rosewater Dish for an eighth time, she needs to improve in all departments.
"I'm just low on matches basically," said Williams, who has opted to double her Wimbledon workload by joining forces with Andy Murray in the mixed doubles even though her build-up was troubled by a knee injury.
"I could feel it but I'm getting there. Just have to... really learn from every single point.
"I'm excited to play with a British icon like Andy... it's going to be an honour to share the court with him. Maybe I can learn a thing or two.
"Usually when I play doubles, it really helps out my singles game. I really need it."
(Reporting by Pritha Sarkar; Editing by Alison Williams and Clare Fallon)
