US Open ’19: As usual, Djokovic, Nadal, Federer lead the way

<p> FILE - In this Sept. 9, 2018, file photo, Novak Djokovic, of Serbia, returns a shot to Juan Martin del Potro, of Argentina, during the men's final of the U.S. Open tennis tournament in New York. Djokovic is seeded No. 1 for the U.S. Open, which he won last year for the third time. (AP Photo/Adam Hunger, File) </p>

Men to watch at the U.S. Open, where play begins Monday: