There was an incredible segment, I believe on 60 Minutes or CNN, of Bennett's final performance in New York City, assisted by Lady Gaga (I don't recall if she sang with him or not). This was whilst he was strongly affected by dementia, and though he showed confusion and cognitive difficulty before the performance, and didn't remember it the next day in a scene shot in Central Park, he absolutely came alive
and hit the notes when on stage.
I've spent a couple of minutes trying to track down the segment on YouTube without luck. It's well worth watching however.
As someone who's not especially a fan of either Bennett or Gaga, I came away profoundly impressed by both.