NEW YORK - Aside from one long, long game, Novak Djokovic kept things quick on a 90-degree afternoon at the U.S. Open on Friday.

The defending champion at Flushing Meadows dropped a 24-point, 15-minute game early in the second set, a tiny blip along the way to wrapping up another easy victory to reach the third round. He beat 112th-ranked Rogerio Dutra Silva of Brazil 6-2, 6-1, 6-2.

"Definitely doesn't feel great, spending 15 minutes for one game in this heat," Djokovic said.

Nevertheless, Djokovic finished with more than twice as many winners, 29, as unforced errors, 14. He needed a little more than 90 minutes all told to make sure he would keep intact his record of never having lost to a player ranked outside the top 100 at a major tournament.

"I didn't know much about my opponent. Never saw him play. So that could have been, you know, a difficulty at the start in order to figure out what his game plan is," Djokovic said. "But I have played well from the start to the end."

Jim Rogash / Getty Images The Week in Sports Pictures The nation grieved for those hurt, killed and affected by the Boston Marathon bombings. After one of the suspects was caught on Friday — following a day-long lockdown and manhunt — sports returned to Boston over the weekend.

The second-seeded Serb has lost a total of seven games through two matches this year at the U.S. Open.

"Obviously, I want to spend (as little) time as possible on the court," Djokovic said. "I have played a lot of tennis, a lot of matches, through the course of this summer."

That includes a runner-up finish to Rafael Nadal at the French Open, and a semifinal loss to eventual champion Roger Federer at Wimbledon, following a title at the Australian Open in January.

Dutra Silva, who never has reached the third round at a Grand Slam tournament, was impressed by Djokovic's play in Arthur Ashe Stadium.

"It's tough to hurt him. I tried, many ways, to hurt him. And actually I can't do that," Dutra Silva said with a smile. "I tried to play baseline. I tried to hit and hit the ball. I tried to play fast. It's so tough."

Djokovic won 37 of 41 points on first serves and never faced a break point all match.

Dutra Silva said that while Djokovic's first serves were hard to handle - the Serb did hit six aces - it was what happened once the ball was in play that was more problematic.

"When I put the ball in the court," Dutra Silva said, "I was in big trouble, you know?"

In other action on Friday, John Isner overcame a rare tiebreaker loss to advance with a second straight four-set win.

The top-ranked American man, Isner beat Jarkko Nieminen 6-3, 6-7 (5), 6-4, 6-3. The ninth-seeded Isner came in 37-13 in tiebreakers this year.

Isner was frustrated by a late let call he thought was incorrect that cost him a point while serving with a 5-4 lead in the second-set tiebreaker. Nieminen went on to win the last three points.

But Isner broke the 42nd-ranked Nieminen four times to reach the third round at the U.S. Open for the fourth straight year.

Isner had 23 aces, 69 winners and 44 unforced errors.

Former champion Juan Martin del Potro beat American Ryan Harrison in four sets in the second round.

The seventh-seeded Argentine won 6-2, 6-3, 2-6, 6-2. The 20-year-old Harrison, ranked 61st, was seeking to reach the third round at a Grand Slam event for the first time.

Each player faced 10 break points, but del Potro converted seven to three for Harrison.

Harrison had 42 unforced errors.

Del Potro, who beat Federer in the 2009 final, couldn't defend his title the next year after right wrist surgery. Del Potro lost in the third round last year.

Lleyton Hewitt is back in the third round of the U.S. Open at age 31 - 11 years after his title at Flushing Meadows.

The Australian two-time Grand Slam champion beat Gilles Muller 3-6, 7-6 (5), 6-7 (5), 7-5, 6-4 in 4 hours, 35 minutes. Hewitt needed a wild card to get in because his ranking had fallen outside the top 100 as he struggled with injuries.