Novak Djokovic says he's in "good position" to complete the first men's singles calendar-year Grand Slam in 52 years by winning four more matches to capture the US Open.

World number one Djokovic rallied past Japan's Kei Nishikori 6-7 (4/7), 6-3, 6-3, 6-2 in three hours and 32 minutes on Saturday to reach the fourth round on the New York hardcourts.