In tennis, an "ace" is when the serve is so perfect that no mortal can even touch it, let alone volley it back. You can find such perfection in all of life's endeavors, even movie casting: Getting Shia LaBeouf to play the part of young John McEnroe in the film Borg/McEnroe, which just had its world premiere at the Toronto International Film Festival, is one such ace. It doesn't mean the whole match is won, but it's something so impeccable that it simply cannot be topped. It's the part he was born to play.
Just in case you don't know who John McEnroe is, he's the man who scandalized my entire family (grandparents, too!) by shouting the F-word on television. I remember it clearly, because my older sister and I repeated it, and even though my mother yelled at us to stop, she was secretly laughing.