NEW YORK—Absorbing an evening of punishment, Diego Schwartzman finally had Novak Djokovic right where he wanted him—completely off the court and stretched out on the defensive.

Then the world No. 1 conjured magic on the move. Djokovic curled a running forehand around the net post, scorching an eye-popping drive down the line into the corner of the court. The shot prompted an astonished gasp from the crowd, including coach Boris Becker, who raised his fist in celebration and gazed up at the big screen grinning to watch the replay for visual confirmation of a stunning shot.

Even when Djokovic was out of position, he turned Schwartzman's best shots into punch lines. The top seed was both overwhelming and entertaining in scoring a 6-1, 6-2, 6-4 triumph to roll into the U.S. Open second round for the 10th straight year.

It was an elegant beatdown beneath the lights. Commanding the center of the court, Djokovic broke minutes into the match for a 2-0 lead before reeling off eight of the next nine points to extend his advantage to 4-0. Moving fluidly, serving with authority, and striking with accuracy, Djokovic simply had too much game for his overmatched opponent. One of Djokovic's only missteps came when he netted a backhand to drop serve in the fifth game. He responded with a four-game run, wrapping up the opening set in 26 minutes, then reeled off eight consecutive points to start the second set.

The 79th-ranked Argentine has assets, but experience isn't one of them. Schwartzman took the court for his first tour-level main-draw match on a hard court armed with an unimposing 3-8 career record. At 5'7" and 140 pounds, Schwartzman showed quick feet, a scrappy disposition, and a sense of humor despite the shellacking. After cracking a forehand winner down the line off a Djokovic smash, Schwartzman cupped his hand to his ear, imploring the crowd to make more noise.

But when you're hitting some sub-75 M.P.H. second serves—as Schwartzman did against the game's most dangerous returner—operating well behind the baseline, and lacking the bite to back up a man who has contested four straight U.S. Open finals, you're bound to get chewed up. Djokovic splattered that forehand around the net post in breaking for 4-1 advantage and slammed an ace down the middle, seizing a two-set lead after just 57 minutes.

The 2011 U.S. Open champion was up a break at 3-2 in the third when he double faulted on break point, gifting a break to the Argentine. By that time, Schwartzman's blue shirt was so saturated with sweat that it clung to his slender frame as if he'd showered fully clothed between sets.

Djokovic broke back at love for 4-3 and never looked back. He concluded the 97-minute mismatch with a forehand net-cord winner, then wrapped his arm around Schwartzman's shoulder at net in a gesture of encouragement.

Djokovic's durability is perhaps his most under-rated strength. The Serbian is contesting his 40th consecutive Grand Slam tournament, which is remarkable when you consider the circumstances of his first career U.S. Open match. A decade ago, the then-97th-ranked Serbian defeated 43rd-ranked Gael Monfils, 7-5, 4-6, 7-6 (5), 0-6, 7-5, in the first career U.S. Open main-draw match for both 18-year-olds. It spanned four hours and two minutes and featured four stoppages of play from Djokovic, who requested the trainer to treat his respiratory and cramping issues. I remember covering that match thinking both could be big winners—if they could stay healthy.

In those days, some opponents regarded Djokovic as physically fragile. But times have changed and Djokovic, who has won 22 of 30 career five-set matches, has undeniable staying power. Next up is Paul-Henri Mathieu. The Wimbledon champion has won five of six meetings with the 32-year-old Frenchman, including winning the last 10 sets they've played.