From the Saturday Stumper

Today’s Newsday Saturday Stumper, by Greg Johnson, is difficult. For me, forty-eight minutes of difficulty. Getting the answer, finally, to the ultra-vague 1-Down, “Development facilitator,” let everything else begin to fall into place.

Two clues that I especially liked: 49-Across, ten letters: “Flat-bottomed vessels.” And 58-Down, three letters: “The tennis US Open is played on it.” Talk about your misdirection! No spoilers: the answers are in the comments.