In the NFL, the quarterback is connected with the coach on the sideline. The coach, shielding his mouth with his play list, calls the play and, through the miracle of modern technology, it arrives in the ear of the quarterback.
This kind of control, in the game of tennis, is not possible. Coaches, sitting in the stands, must resort to hand signals or brief oral observations. Both of which could incur a penalty if the referee deems they cross the line.
In Davis Cup, that cauldron of international chaos, hands-on coaching is allowed. World Team Tennis is the only other venue that encourages audience participation. The coach sits on the sideline and can discuss strategy at any juncture. Does it make any difference?
"I think it does," said U.S. Davis Cup captain Patrick McEnroe. "I think you can see tendencies going on that the player, maybe, is not seeing. It's not extreme, but it can make a difference. Little things that can help. In certain matches, those little things can make a huge difference."
McEnroe knows this from firsthand experience. He has the United States team in the semifinals. Andy Roddick, the Bryan Brothers and a singles player to be named later will take on Belarus in Charleston, S.C., Sept. 24-26. The winner will meet the winner of Spain vs. France in late November.
The United States leads all competitors with 31 Davis Cup championships but has not won since 1995. McEnroe followed his brother John as captain in December 2000 and the team has rallied around him. McEnroe has reached out to the young American players and made it a point to discuss the finer points of the game. It isn't all clapping and cheering.
"One of the matches I remember was when Pete Sampras was playing Karol Beck [in 2002]," McEnroe said. "Pete wasn't playing great, but I noticed when he moved in close on Beck's serve, he panicked. God knows, I'm not dictating strategy to Pete Sampras, but he'd down a game in the fourth set with one last chance to break him. I mentioned it to him and, sure enough, the guy double-faulted twice.
"Sometimes, it's just having another set of eyes. In Davis Cup, you can translate that into results."
Greg Garber is a senior writer for ESPN.com.