While Japan is enjoying some success in its battle with speculators targeting the enfeebled yen and the central bank’s stubborn grip on yields, more tests lie ahead.

After months of jawboning turned into concrete action in the foreign-exchange market, speculative positioning has been pegged back at least in the currency space. Together with a favorable reduction in demand for the greenback, that has kept the yen well away from the key ¥150 per dollar mark in recent weeks, though U.S. inflation data Thursday is an imminent threat.

"Japan’s authorities aren’t likely letting their guard down, but it’s looking increasingly likely that they’ve persevered and pulled themselves out of the woods,” said Jun Kato, chief market analyst at Shinkin Asset Management in Tokyo. "They’ve also gotten lucky with the external environment, which seems to be changing.”