I feel like I'm playing chess with an invisible opponent and he just made a move.

Hill was no coward but he didn't like fighting against unknown and invisible opponents.

She said it fiercely, as if arguing against some invisible opponent.

There was no point in fighting against an invisible opponent.

It was curious how she seized the occasion to argue with some invisible opponent.

Singh's fists came up as if he were ready to fight an invisible but powerful opponent.

Until Koch came up against a nearly invisible opponent, that is.

In a moment, he was fighting an invisible opponent.

He struggles, as though wrestling with a pinned, invisible opponent.

His breath came and went quickly,-- he seemed to challenge some invisible opponent.