EU President-elect Jean-Claude Juncker says he "fought like a lion" to keep Greece in the euro single currency, visiting Athens on his first trip since winning the election for Europe's highest office.

The former eurogroup chairman on Monday praised Greek efforts to improve public finances but declined to comment on options being considered by eurozone countries to make Greece's runaway national debt sustainable.

Juncker, who succeeds, Jose Manuel Barroso in November, also declined to give details on his choices for the next European Commission.

Greece has promised to end a crippling recession this year after being bailed out from the brink of bankruptcy four years ago, with billions in rescue loans from the eurozone and the International Monetary Fund.