NEW YORK -- Novak Djokovic admittedly got a bit distracted.
He was unaware of the US Open rule about time allowed between points. He barked in the direction of his entourage -- among the only people in the Arthur Ashe Stadium seats. In the end, though, he did what he always does in 2020: win.
"I lost my focus,'' Djokovic said afterward. "Kind of got stressed out a couple times. Screamed.''
The No. 1-ranked Djokovic began his bid for Grand Slam title No. 18 on Monday by extending his season start to 24-0 with a 6-1, 6-4, 6-1 victory over Damir Dzumhur at Flushing Meadows.
"Do I want to keep the streak going? Of course I do. Am I thinking about it as a priority No. 1 every single day? No,'' said Djokovic, who opened 2011 with a 41-0 mark. "It's there, and of course it's an additional motivation for me. It actually fuels me to play even stronger, play even better, I think, bring the right intensity every match.''
Although Djokovic won handily, Day 1 on the men's side saw some upsets, as Steve Johnson knocked off 16th-seeded John Isner 6-7 (5), 6-3, 6-7 (5), 6-3, 7-6 (3) to advance to the second round after a match that stretched 3 hours, 50 minutes. Earlier in the day, No. 9 seed Diego Schwartzman blew a two-set lead and was knocked out in the first round. He led 2-0 but appeared to have problems with cramps as he lost to Cameron Norrie 3-6, 4-6, 6-2, 6-1, 7-5 in a match that took nearly four hours.
During a prematch TV interview, Dzumhur said of Djokovic: "Hopefully, he is not 100 percent.''
That was probably a reference to the way Djokovic dealt with neck and stomach issues during his run to the Western & Southern Open title last week on the same hard courts being used for the US Open.
Djokovic played a three-set semifinal Friday and a three-set final Saturday.
The 48 hours before facing Dzumhur, who has been ranked as high as 23rd and now is 109th, apparently were enough for a full physical recovery for Djokovic.
"I felt good on the court today,'' said Djokovic, whose 17 Grand Slam tournament titles trail only that of Roger Federer, with 20, and Rafael Nadal, with 19.
Neither of those rivals is in the US Open, which is only part of the reason Djokovic is an overwhelming favorite to win what would be his sixth title in a span of eight major tournaments.
One minor hiccup during his opening match had to do with the way the serve clock is being implemented at this US Open: Chair umpires are starting the 25-second countdown much sooner than they were in the Western & Southern Open.
"Why did you start it?'' Djokovic asked chair umpire Damien Dumusois, noting that in the previous event, players got more time to collect their towels between points.
Dumusois said the pace is supposed to be quicker at the US Open, to which Djokovic replied, "You do it here different? Why? There is no explanation? ... Thanks for letting us know.''
After the match, Djokovic said, "No one really brought it to my attention. The lack of communication is something that worries me once again. I mean, that's something that really upset me. ... We've played in the certain tempo, so to say, got used to it during the Western & Southern tournament, which just ended two days ago. Two days later, we have a different rule that was just not communicated to us. That's something that I found just not acceptable, not fair.''
Also Monday, seventh-seeded David Goffin held on to beat Reilly Opelka 7-6, 3-6, 6-1, 6-4, and 19th-seeded Taylor Fritz topped Dominik Koepfer 6-7, 6-3, 6-2, 6-4.
The 6-foot-11 Opelka is tied as the tallest top-50 player in ATP rankings history. Fritz, the second-highest ranked American, was the 2015 US Open boys' singles champion.
The Associated Press contributed to this report.