NEW YORK -- Novak Djokovic shook off a back problem early in the match to beat Cam Norrie on Friday night, and at 38, he became the oldest man to reach the fourth round of the US Open since Jimmy Connors was the same age in 1991.

Djokovic pulled away to win 6-4, 6-7 (4), 6-2, 6-3, helped by hitting 18 aces in what he said was his best serving performance of the tournament.

The 24-time Grand Slam champion seemed in jeopardy of matching his earliest exit in Flushing Meadows when he needed medical attention near the end of the first set. But he recovered to reach the last 16 of a major for the 69th time, matching Roger Federer for most ever, and will next face German qualifier Jan-Lennard Struff .

The No. 7-seeded Djokovic wouldn't reveal specifics of his injury in his on-court inter

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