Novak Djokovic beat Britain's Cameron Norrie in four sets on Friday to became the oldest man to reach the US Open last 16 since Jimmy Connors in 1991.
The 38-year-old Djokovic earned a record 192nd Grand Slam match win on hard courts as he defeated Norrie 6-4, 6-7 (4/7), 6-2, 6-3, staying in the hunt for an unprecedented 25th major title.
"Coming into any match you really want to win in straight sets without any drama but obviously that's not possible," said Djokovic.
"It's good that I get tested. I obviously haven't had any matches since Wimbledon.
"I'm still trying to find my rhythm and my groove on the court."
Djokovic has now defeated Norrie in all seven of their meetings and is through to the fourth round in New York for the 16th time in his career.
On Sunday he will play German veteran Jan-Lennard Struff, who has already knocked out two seeds in Holger Rune and Frances Tiafoe, AFP reported.
A lone break of Norrie's serve in the seventh game was enough for Djokovic to pocket the opening set, but not before he summoned the physio to work on his lower back.
The Serbian star headed off court for a medical timeout and returned to serve out the first set.
"It's all right, you have some ups and downs. You don't want to reveal too much to the rivals listening," said a grinning Djokovic, refusing to elaborate on the nature of his back issue.
He had the chance to make further inroads in the second set but Norrie saved three break points and then pinched the tie-break from his smoldering rival who double-faulted on set point.
Norrie took that momentum into the third set with an early break, drawing an emphatic response from Djokovic as he responded by winning four straight games.
Djokovic soon made the set his and another break to start the fourth set gave him the cushion required to close out a milestone victory.
