But despite the scoreline of 6-4, 7-6(3), 6-1, there could be major concerns for the world No. 1 and defending champion.

Djokovic, with pain in his left shoulder, was in initial trouble against the Argentine.

The 16-time major champion, who had visits from the trainer multiple times, could be seen grimacing early and was shaking out his left arm during the match.

Djokovic said to ESPN on court after the match that the shoulder was affecting his serve and backhand, but he didn't want to elaborate further on the extent of the injury.

"I was definitely tested," Djokovic said to ESPN. "This is something I've been carrying for quite a while now."

'I didn't know if I would be able to finish the match'

In his post-match press conference, Djokovic said the injury has bothered him now for almost a couple of weeks.

"I've been experiencing some days of higher intensity of pain, some days less," he said. "It has been really fluctuating a lot, going up and down."

He also admitted he wasn't sure if he would make it through the match.

"It was not easy to play with this kind of sensation, to be honest," Djokovic said. "I did not experience that too many times in my career.

"I was also lucky to find my way back in the second set and to win in the straight sets. I had obviously, you saw, a medical timeout. At changeovers, I tried to use within the rules as much as I can physiotherapy and medical help. That has definitely helped me stay in the match.

"The way it has started for me, especially midway through the first set, I didn't know if I would be able to finish the match. I'm really glad I have.

"I'm going to assess this injury tomorrow even more with further consultations with experts in sports medicine. I'm hoping that in two days' time I will be able to play pain-free, if that is possible."

'It wasn't easy, obviously, playing with the pain'

Trailing with Londero up a break at 3-2, Djokovic was able to break back and then saw the trainer. He was able to close out the set, but he sought the trainer again right after doing so.

The Serbian's struggles continued, with Djokovic down a double break to start the second set. Things started looking up when he rattled off five straight games, but a shaky service game put Londero back in it and led to a tiebreak. After Djokovic came out on top, the trainer came back out on court to work on his shoulder again.

After trading breaks to open the third set, Djokovic pulled away.

With the win Wednesday, Djokovic ties his childhood idol, Pete Sampras, for the fifth-most US Open wins (71) in the Open era.

"It wasn't easy, obviously, playing with the pain, but you have to find a way to fight and hope that you'll get some lucky shots," Djokovic said to ESPN. "You get some opportunities when they're presented, you have to step in and take them. That was the case. I had some luck, as I said, in the second set, to come back.

"I'll move on. It's not the first time that I'm facing this kind of adversity so to say, or a challenge. It is what it is, and I'm just grateful to be on the court."

In February 2018, Djokovic underwent surgery for an elbow injury that had troubled him for two years.

Djokovic, who will have the day off Thursday, will next face either American Denis Kudla or fellow Serbian and No. 27 seed Dusan Lajovic. Those two won't have the luxury of rest like Djokovic; their match, as so many others on Wednesday, got washed away by rain. They're scheduled to now play in what will be a busy Thursday, with the winner to face Djokovic on Friday.

"I'll probably freeze my arm for 48 hours, not do anything with it, and see what happens," Djokovic said to ESPN with a laugh.