NEW YORK (AP) — Novak Djokovic was cruising along with a big early lead in his U.S. 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.”

Djokovic was ahead 4-0, 15-love when he hit a nicely angled volley winner to go up 30-love in that game. The 24-time Grand Slam champion immediately grabbed at the back of his neck and started turning his head.

The 38-year-old Djokovic kept trying to stretch his neck or flex his right shoulder betwe

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