NEW YORK — Novak Djokovic was cruising along with a big early lead in his US Open fourth-round match on Sunday night when his neck started bothering him. That didn’t stop Djokovic from dominating 144th-ranked qualifier Jan-Lennard Struff 6-3, 6-3, 6-2 to reach his record 64th Grand Slam quarterfinal. “What gives me a lot of encouragement right now, and positivity, is the way I played tonight,” Djokovic said. “Best performance of the tournament so far. Hopefully I can keep that going.” READ: US Open: Novak Djokovic shakes off back issue, enters fourth round Novak Djokovic reflects on being the oldest […]... Keep on reading: Novak Djokovic's neck bothers him in US Open win