“It’s the punch you don’t see coming that knocks you out” (or, “the punch you don’t see is the punch that hurts the most”) is an old boxing adage. “The punch you don’t see is the punch that hurts” was said by light heavyweight boxer Willie Pastrano (1935-1997) in June 1962. “And it’s the sneaky punches—the ones that you don’t see coming—that do the most damage” was in a New York (NY) Times story in September 1962. “The punch that puts you to sleep is not so much the hard punch as the punch that you don’t see coming” was printed in a 1971 book about Muhammad Ali by José Torres (1936-2009); Torres had won the light heavyweight championship by defeating Pastrano in 1965. It’s likely that Pastrano at least popularized the saying in 1962.
 
“It’s the punch you don’t see coming that knocks you out” has also been used as a business and investment aphorism.
 
 
19 June 1962, The Times-Picayune (New Orleans, LA), sec. 2, pg. 4, col. 3:
Pastrano Is a Man
With New Ambition
“Draw with Moore Gave Me a Goal”
By BUDDY DILIBERTO
Willie Pastrano, whose boxing career appeared ended just a year ago, came “home” Monday night full of new ambition spurred by his recent draw with Archie Moore.
(...) (Col. 4)
“I fought him my way—not his. I made him come to me. I would have been a sucker to go after him. The punch you don’t see is the punch that hurts. I made sure I saw all his punches.”
 
21 September 1962, New York (NY) Times, “Patterson Says Size Is No Factor: Liston’s Weight, Reach Fail to Impress Champion” by Robert L. Teague, pg. 21:
And it’s the sneaky punches—the ones that you don’t see coming—that do the most damage.
 
Google Books
Sting Like a Bee:
The Muhammad Ali Story
By José Torres
New York, NY: Abelard-Schuman
1971
Pg. 44:
So we arrive at one knockout of a conclusion: the punch that puts you to sleep is not so much the hard punch as the punch that you don’t see coming. But the knockout process is more complicated than that. It’s a matter of will.
 
Google Books
Three Variations on the Theme of Harm:
Selected Poetry and Prose
By Douglas Oliver
London: Grafton
1990
Pg. 60:
The boxing profession generally has believed the Cus d’Amato doctrine reported in Jose Torres’s classic on Ah: ‘It’s the punch you don’t see that knocks you out.’
 
Google Books
The Bastard on the Couch:
27 Men Try Really Hard to Explain Their Feelings About Love, Loss, Fatherhood, and Freedom
Edited by Daniel Jones
New York, NY: HarperCollins
2005
Pg. 9:
But as any boxing trainer will tell you, it’s always the punch you don’t see that knocks you out.
   
Zero Hedge
The Knockout Blow People Will Not See Coming
Submitted by Tyler Durden on 04/06/2013 19:45 -0400
Submitted by Simon Black of Sovereign Man blog,
(...)
Some of you may know that I was a competitive fighter for a number of years. I can personally attest, and any boxer will tell you, that it’s the punch that you don’t see coming which knocks you out.