Don Bradman's average is the most astonishing stat in sport, still.* But Isner vs Mahut joined it in the list of things that occupy entirely their own statistical reality. I'm trying to think of other records like them. There must be a few.
* Bradman's test average score was 99. In the history of test matches, only three other players who have completed twenty innings have averaged over 60, and they all averaged less than 61. Only about 40 others have averaged more than 50.