ball boy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A male attendant on a tennis court who collects the ball when it is out of play.
• noun A boy who is in charge of the extra balls used in practice or those that are out of play, especially in baseball.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun sports A person, usually male, responsible for retrieving balls from the area of play, and returning them to the players.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a boy who retrieves balls for tennis players