The Davis Cup will experiment with a shortened format next year despite national federations voting against it.
At the International Tennis Federation's AGM in August, a proposal to change singles matches from best of five sets to best of three was rejected.
But ITF members voted to give the board of directors the authority to make changes to the competition on a trial basis, and they have acted quickly.
All matches in Group I and II across the different zones will be played over best of three sets while ties will be reduced from three days to two.
Those changes will not be implemented in the elite World Group, but another trial, increasing teams from four to five players, will come in while fewer dead rubbers will be played.
The ITF hopes format changes can restore some of the lustre that the competition has lost in recent years.
The success of the recent Laver Cup, which attracted many of the players who have skipped Davis Cup, appeared to deal another blow, while plans for a team event on the ATP Tour are also progressing.
ITF president David Haggerty said: "We are as determined as ever to enhance our flagship team competitions for players, fans, host nations, broadcasters and sponsors.
"The nations voted to give the board the flexibility to trial changes, and the board has acted quickly to make this happen in 2018.
"The board felt it was appropriate to trial the two-day, best-of-three-set format at Zone Group level. Should the trial prove to be a success, we will be in a position to consider whether it should be applied for the World Group as well after 2018.
"Our priority remains to attract the top players, and these trials address some of the challenges that these players face in an increasingly crowded tennis calendar."