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 rig…