It stands to reason that the term “dark horse” originates from the world of horse racing.
For those betting on the outcome, it meant that setting odds became difficult when a horse was not known to gamblers.
Today, in the world of sports or politics, it signifies that a person who was not well known emerged as a winner––an unexpected winner.
So while Juan Martin del Potro may not be a "favorite" entering the 2011 U.S. Open, the Argentine cannot be regarded as a “dark horse” because he is well-known in Flushing Meadows.
Del Potro won the US Open title in 2009.
True "dark horses" are not widely known by the general viewing public because they are relatively new on the tennis scene.
These young players have not made much of a name for themselves––yet.
To date, there are six young guns ready to make their way to the top of the men’s game.
They will become the "dark horses" in the upcoming US Open, representing the future of tennis on the men's side of the draw.