The U.S. Open begins Monday at Flushing Meadows. Here are five men to watch during the final grand slam tournament of the season:

Novak Djokovic

The defending U.S. Open champion was stunned last week in the Cincinnati tune-up, but it’s hard to bet against the Serbian capturing his fourth title in Flushing and third Grand Slam of 2019. The best defender in tennis history is just a relentless, resilient force and showed it again with his epic Wimbledon five-hour, five-set tiebreaker squeaker over Roger Federer. Another Slam title would put him three away from tying Federer’s record of 20. Djokovic has advanced to the Open finals eight times.

Roger Federer

It’s hard to believe the last time Federer won the Open was 2008. At age 38, Federer still is a factor, but has grown a mental block here after once being Flushing king five straight years (2004-08). The courts in recent years just haven’t played fast enough for the Swiss Maestro’s all-court game, and he looked in Cincy like he hasn’t recovered from blowing championship points in the Wimbledon heartbreaker.

Rafael Nadal

With Andy Murray all but out of the picture, the “Big Three” have combined to win the past 11 straight Grand Slam titles — and 54 of 65 since 2003. This is hardly Nadal’s favorite place, and his knees have not always cooperated here — including in last year’s semifinal vs. Juan Martin del Potro. But the lefty Spaniard recently won the Open tune-up in Montreal and is always going to be a threat. Maybe this can finally be the year Nadal and Federer meet at the Open for the first time — a stunning statistical anomaly.

Daniil Medvedev

The 23-year-old from Moscow is the tour’s hottest player this summer after advancing to the finals in Washington and Toronto before winning in Cincy by toppling Djokovic. He is a fantastic mover for someone his height (6-foot-5) and frustrates guys by getting to lots of balls. Plus, he keeps improving on his serve. His ranking has climbed to No. 5.

Reilly Opelka

The 6-foot-11 American has a better chance of winning the Knicks’ backup center job than his first Grand Slam title, but it’s time to take him seriously as next future U.S. men’s hopeful. The tallest player in tennis history will draw crowds here. The 21-year-old from West Palm Beach, Fla., won the New York Open on Long Island in February to trigger a solid year that included upsetting Stan Wawrinka at Wimbledon to make the third round. Now ranked No. 41, he’s been using John McEnroe’s fitness man and his movement and health has been golden.