Djokovic had been chasing an 18th Grand Slam title at the Billie Jean King US National Tennis Center.
The Serb, who had previously been suffering with a shoulder injury, was visibly frustrated after being broken in the first set as the Spaniard took a 6-5 lead.
Djokovic rushed to the back of the court to check on the lineswoman, then spoke at length to tournament officials on court before he was disqualified.More news: Tenet Earned $100 Million At The Box Office Worldwide
With this default, Djokovic's unbeaten run in 2020 has come to an end.
While there was no malice in Djokovic's strike, he did himself no favors blowing off the mandatory post-match press conference rather than explaining his side of the story.
It is Djokovic's second straight US Open fourth-round exit-he retired from his 2019 round of 16 match against Stan Wawrinka-and makes this unpredictable US Open wide open.More news: Trump calls on Iran to not execute wrestling star Navid Afkari
Under the rules of the game there is no option other than a default and despite Djokovic's pleadings that he had not meant to hit the official, he was eventually disqualified. She required medical treatment and left the court.
Not long before the incident, Djokovic had fallen to the court and landed awkwardly, with most of his weight resting on his left wrist and arm.
In the prior game, he wasted three consecutive break points and after the last, which Carreno Busta won with a drop shot, Djokovic whacked a ball off a courtside advertising sign. Moments later the 2017 semifinalist watched Djokovic's default and advanced to a quarterfinal against either seventh-seeded David Goffin or 12th-seeded Denis Shapovalov.More news: Third phase of Sputnik V trials starts in Russian Federation