A 1910 advertisement for Coca-Cola featuring St. Louis Cardinals first baseman Ed Konetchy:
“He likes it, he says, not alone for its deliciousness and its refreshing qualities, but because it relieves fatigue of body and brains and it is the best beverage for quenching thirst he’s ever tried.
“Such an endorsement from such a ballplayer should recommend Coca-cola to you, whether you be amateur or professional.”
Two years later, Konetchy shared his theories on batting and batting slumps, with The St. Louis Globe-Democrat:
“The ordinary person cannot for the life of him reason out why a ballplayer should be able to hit one part of the season and suddenly take a slump and act like a novice at the bat. To one familiar with the playing of the game of baseball, the reason for this was a well-known fact.
“Good hitting, considering of course a natural player, generally depends upon the physical and mental condition of the man. When he is in good shape, his eye is clear and his brain works quickly; when he is out of condition his mind is dull and he loses his eye for the ball.”
[…]
“The other day as were leaving the field of St. Louis, I heard a fan, referring to one of our players, remark: ‘They ought to bench that fellow, he can’t hit anything.’ As a matter of fact, the man to whom he referred was one of the best stickers of our team and had merely been up against a little hard luck. For three or four days, I have watched this player and almost every time he came to bat he had met the ball squarely but could not seem to place it into fair territory. This is a fact that is not taken into consideration by a great many baseball lovers. They judge a man’s ability to hit by reading the scores in the next day’s papers, not at all stopping to think that a man who has no hits credited to him may have done far more towards winning the game than the player who annexed two or three safeties.”
Baseball History Daily 
I found this advertisement in my attic,
Very cool.