Though his facial expression didn't change, the Doctor was relieved that the Master hadn't started to gloat - admitting he was wrong was bad enough, he didn't need his to have somebody rub it in. But it was a relief off his shoulders to know there was somebody who knew where he was coming from and understood.
The Doctor looked over his shoulder at the Master, empathizing with his situation. "You do make it out, though. I told you, I met your future regeneration. You're bonkers, but you survive." He nodded and then added, "And at least here, you have a second chance. What without the..." he waved his index finger at his own head, to indicate the drums which the Master had been suffering from.