More used to mowing down opponents on the All-England club's manicured lawns with five singles titles to her name, the dominant player in women's tennis was left to explain how she was ambushed by German Sabine Lisicki on Monday.
The unheralded 23rd seed took the biggest scalp of her career when she edged a gripping-fourth encounter 6-2 1-6 6-4 to reach the quarter-finals.
With second and third seeds Victoria Azarenka and Maria Sharapova heading home in the first week, talk that the American top seed had another grand slam title at her mercy proved wide of the mark.
"I've said this, I don't know if you've heard, but she's a great grasscourt player," Williams told a news conference.
"You know, c'mon, guys, let's get with it. She's excellent. She's not a pushover. She's a great player."
Lisicki enjoyed a run to the Wimbledon semi-finals in 2011 before losing to runner-up Maria Sharapova and her powerful serve put Williams under the kind of pressure she rarely gets in the early rounds of a tournament.
"It's not a shock. Her ranking has no effect on what she should be," Williams added.
"She should be ranked higher. Especially on grass she just has, you know, a super, super game to play well. I'll just have to go back to the drawing boards and figure out a way how to win this match the next time."
Getting to the root of her loss, Williams said that having battled back from a disappointing opening set to breeze through the second and go 3-0 up in the decider, she lost confidence on her serve.
"I felt that I was on the verge of winning. At that point I just was physically unable to hold serve. My first-serve percentage was going down," she said after Lisicki broke in three successive times to move ahead 5-4.
"You have to be ready and willing to hold your serve. I wasn't willing or able, probably didn't even want to hold my serve today."
Her demise leaves fourth seed Agnieska Radwanska as the highest ranked player left in the last eight, but in a tournament now wide open Williams tipped compatriot Sloane Stephens, a player who seemed determined to upset her illustrious rival earlier this year.
The 20-year-old, who came through in three sets against Puerto Rican Monica Puig on Monday, beat Williams in the quarter-finals of this year's Australian Open and later criticised her in a magazine interview.
Some scathing comments caused more bemusement than hurt for Williams who refused to get drawn into a war of words.
Showing there was no lingering resentment, Williams said: "I think Sloane has a really good chance of winning. She has a great draw. I think she can take it. It would be really nice to see her win."