Ok you're joking....
But still... Maybe you need to explain yourself a little better. So..Bagels help pro players play better. Why?
(Looking forward to this answer
I could give you some BS long winded answer... like how the word bagels and tennis share a striking parallel! (2 vowels and 4 consonants - WoW!) Or like how bagels strangely resemble the shapes of some galaxies... and thus inherently gives the eater the wisdom and courage of the stars. Instead though I will only tell you the boring and honest truth.
For a tennis player the simple answer is this:
If a tennis player eats a bagel within a couple of days before playing in a tournament, while on the tennis court they will subliminally be thinking of bagels and how DELICIOUS they are. This in turn puts their mind off the match, but they don't realize it. Before said player knows it, they have lost the first set. The magic kicks in when the player on the other side of the net subliminally believes that the set was given to him - that the bagel eating player was not giving it 100%. This is where the magic of eating the bagel kicks in. The opponent of the bagel muncher, due to his subliminal belief, cannot sustain the same level of intensity or consistency. This coincides with the fact that after a set of tennis the bagels effects wear off (This can sometimes take slightly longer or shorter than one set mind). The interesting thing is where the transference of the bagels power has suddenly dissipated to... the answer? The racquet. All of a sudden the 2 days previous bagel devourer can hit the ball 20 percent harder, with 20 percent more angles.. has 20 percent more touch etc.
This is why James, eating a bagel almost guarantees victory, and in the case of Roger Federer, a 100 percent assured victory.