"The serve was invented so (that) the net can play” is a tennis joke by comedian Bill Cosby that has appeared in many quotation collections. The joke first appeared in Cosby’s book, Bill Cosby’s Personal Guide to Tennis Power (1975).
Wikipedia: Bill Cosby
William Henry “Bill” Cosby Jr. (born July 12, 1937) is an American comedian, actor, author, television producer, educator, musician and activist. A veteran stand-up performer, he got his start at the hungry i in San Francisco and various other clubs, then landed a starring role in the 1960s action show, I Spy. He later starred in his own sitcom, The Bill Cosby Show. He was one of the major performers on the children’s television series The Electric Company during its first two seasons, and created the educational cartoon comedy series Fat Albert and the Cosby Kids, about a group of young friends growing up in the city. Cosby also acted in a number of films.
OCLC WorldCat record
Bill Cosby’s Personal guide to tennis power : or, Don’t lower the lob, raise the net
Author: Bill Cosby; R M Meek
Publisher: New York : Random House, 
Edition/Format: Book : English : 1st ed
29 June 1975, Star-News (Pasadena, CA), “Tennis Power? Cosby’s For It” reviewed by Dan Meyers, pg. B6, col. 6:
And what other group can better comprehend Cosby’s assertion: “The serve was invented so the net can play. If by some magic (the serve) goes over, you’re so busy marvelling at your work, you let your opponent’s return go by.”
14 August 1975, Milwaukee (WI) Star, “Night Scene,” pg. 10, col. 1:
Here’s another laughter from the new book of Bill Cosby, who co-stars with Sidney Poitier in “Let’s Do It Again.” The book is “Bill Cosby’s Personal Guide to Tennis Power, Or, Don’t Lower The Lob, Raise the Net” (Random House). “The Serve: The serve was invented so the net can play. Generally, the net is a very good player with a fine sense of humor. It blocks all first serves, nips all second serves and laughs at you. And why not? It’s very funny what you look like when you serve.”
12 May 1982, St. Albans (VT) Messenger, “Fat Albert - OK” (UPI), pg. 4, col. 3:
Serve: The serve was invented so the net can play.
Last Minute Speeches and Toasts
By Andrew Frothingham
Franklin Lakes, NJ: Career Press
“The serve was invented so that the net can play.” —Bill Cosby
The Mammoth Book of Zingers, Quips, and One-Liners:
Over 10,000 Gems of Wit and Wisdom, One-liners and Wisecracks
Edited by Geoff Tibballs
New York, NY: Carroll & Graff
The serve was invented so that the net could play. BILL COSBY