Daniil Medvedev, the fourth seed, struggled in his third round matchup against Filip Krajinovic, losing the third and fourth sets in spectacular fashion. Medvedev cycled between yelling at himself in Russian, his coach in French, and Krajinovic’s well placed volleys in English. In the fifth set, Medvedev settled back in and won six straight games.For McDonald to pull off an upset, he will need to exploit Medvedev’s natural volatility. Medvedev should be able to keep McDonald at bay if he can keep calm and use his varied shots to pull the American around the edges of the court.Rod Laver Arena | 11 p.m. SundayRafael Nadal vs. Fabio FogniniRafael Nadal, the No. 2 seed, has struggled with a small back injury throughout the first week of the Australian Open. However, this has not stopped him from rolling past his opposition without dropping a set. Nadal’s powerful topspin shots have consistently pushed his opponents around the court, depriving them of the time necessary to impose their own ideas upon a rally.Fabio Fognini, the 16th seed, has had a roller coaster week. He struggled in a five-set contest against a fellow Italian, Salvatore Caruso, but then dispatched the 21st seed, Alex de Minaur, in just three sets. Fognini, who won a doubles title at the Australian Open in 2015, has been to the round of 16 in Melbourne four times, and will have a difficult time overcoming Nadal.Rod Laver Arena | 3 a.m. MondayAshleigh Barty vs. Shelby RogersAshleigh Barty, the world No. 1, has moved through to the fourth round without dropping a set. In her third round victory over Ekaterina Alexandrova she played smart tennis, not going for big shots and allowing Alexandrova to overplay and extracting 30 unforced errors.Shelby Rogers, an unseeded player, has reached two major quarterfinals, but has never won a WTA tournament. Her inconsistency on tour can partially be blamed on ruptured cartilage in her knee, which required surgery in 2018.Barty and Rogers faced off in the quarterfinals of the Yarra Valley Classic last week, with Barty winning in a third-set tiebreaker. For Rogers to reverse her fortunes, she’ll need to play aggressively without over-hitting, a tough needle to thread.