MELBOURNE, Australia—As Daniil Medvedev sat courtside trying to re-hydrate during a changeover, an image of nine-time Australian Open champion Novak Djokovic flashed up in the stadium behind him. It was in the third set Saturday, and it was like the absent No. 1 was looking over the shoulder of the player who is effectively the No. 1 seed at the year’s first Grand Slam tournament. Medvedev, who lost last year’s Australian Open final to Djokovic but avenged that with a victory over the Serb for the U.S. Open title, reached the fourth round for the fourth straight year at Melbourne Park with a 6–4, 6–4, 6–2 win over Botic van de Zandschulp. The 25-year-old Russian was a conspicuous fan favorite on Margaret Court Arena, too, two days after being unsettled by the boos and jeers of a parochial crowd on Rod Laver Arena when he ended the run of mercurial …