Novak Djokovic warned that his Grand Slam quest faces a tough challenge from Italian sixth seed Matteo Berrettini in the US Open quarter-finals as they meet in a third consecutive Slam.

The 34-year-old Serbian advanced within three matches of completing the first calendar-year Grand Slam in 52 years yesterday morning (Singapore time) by fending off 99th-ranked American Jenson Brooksby 1-6, 6-3, 6-2, 6-2.