NEW YORK (AP) — Past champion Novak Djokovic returns to the U.S. Open, and No. 2 seeds Roger Federer and Caroline Wozniacki are in action on a sweltering second day at Flushing Meadows.

The No. 6-seeded Djokovic faces Marton Ducsovics in his first match at the U.S. Open since losing to Stan Wawrinka in the 2016 final. The Serbian missed last year because of an elbow injury but is one of the favorites after winning Wimbledon in July and beating Federer in the final at Cincinnati this month.

Djokovic could face a quarterfinal matchup with Federer, the five-time champion who meets Yoshihito Nishioka of Japan in a night match.

Wozniacki is already the highest-seeded woman remaining after top-ranked Simona Halep lost on Monday.

A heat advisory is in effect in New York through late Wednesday, with temperatures in the mid-90s and heat index values in excess of 100 degrees (38 Celsius).

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