Mixed Up

After months of handwringing, the crowdpleasing U.S. Open mixed doubles shindig kicked off.

NEW YORK — The first day of the U.S. Open mixed doubles extravaganza is in the books, and at least on site here in Flushing Meadows, reviews for the new concept have been overwhelmingly positive.

Thousands packed into both Arthur Ashe Stadium and Louis Armstrong Stadium for the 12 matches on Tuesday; Armstrong, which was free to enter, was especially ful…