Tsitsipas began the season at the Hopman Cup alongside Maria Sakkari, making them the first team to represent Greece at the event in 17 years. They finished in a three-way tie for first in their round-robin group, having won their ties against the United States and Switzerland after being upset in their opening tie against Great Britain. They were ranked second on the tiebreak criteria and did not advance out of the group. Tsitsipas's only singles win was against Frances Tiafoe, while one of his losses was against Roger Federer. At the 2019 Australian Open, Tsitsipas reached his first major semifinal, despite having won just five Grand Slam singles matches in total the previous year. All five of his wins at the event came in four sets. He upset world No. 3 and defending champion Roger Federer in the fourth round, saving all twelve of the break points he faced in a rematch of their Hopman Cup encounter earlier in the month. After continuing his run against Roberto Bautista Agut, he easily lost to No. 2 Rafael Nadal, winning only six games. Nonetheless, his performance helped him climb to a career-best ranking of No. 12 in the world to become the highest-ranked Greek player in history.