Who are the power countries of tennis? Are they the ones who perform well in the Davis Cup regularly?
The Davis Cup site has the rankings of countries displayed here—it is a good indicator of the depth of the countries' strength in the game.
Though most of the top players compete for their country, it is not a guarantee that everyone will sign up.
Hence, though the list gives a good feeler for the power list, this might be 10-20 percent off the mark.
Who are the players who present the best chances from their country in terms of potential ability to break into the Top 10 or even Top 20 list?
The list contains fifteen countries as present on the Davis Cup listing and sorted in the descending order of the (top) player rankings on the ATP tour.
Because of this cut-off, Great Britain drops off the list—their ranking is a lowly 35.
Similarly, teams like Kazakhstan and Israel drop off since none of their players appear in the Top 50 of the ATP Tour.
Let me know if you can add other countries and their representatives—I will add to the list.