years old Corentin Moutet in a third-set tie-break. Next, he reached the quarterfinals at New York Open losing to second seed Sam Querrey. After a string of tight first round losses at Delray Beach Open, Indian Wells Masters and Miami Open (tennis), the U.S. Men's Clay Court Championships in Houston saw Karlović reach the semifinals with wins over Japanese Taro Daniel, Denis Kudla and 4th seed Nick Kyrgios. In the semifinals, he lost to eventual runner-up Tennys Sandgren in two close tie-break sets. At Wimbledon, he reached the second round. At the US Open, he was defeated in the second round of qualifying. In October, he became the oldest champion of an ATP Challenger tournament, winning the Calgary Challenger at 39 years and seven months. 2019: Oldest ATP finalist since 1977 At the Maharashtra Open Karlović beat teenager Félix Auger-Aliassime in first round. The age gap of 21 years and 6 months between the two players was the largest on the ATP Tour since 2011. Karlović and Auger-Aliassime were the oldest and the youngest players in the Top 200 ATP rankings at the beginning of the 2019 season. He went on to make the final, at 39 years and 10 months becoming the oldest finalist on the ATP Tour since 43-year-old Ken Rosewall won the 1977 Hong Kong championship. With his victory over Matthew Ebden at the Indian Wells Masters, he became the first player aged over 40 to win an ATP Tour match since Jimmy Connors in 1995 in Halle. In addition, he is the oldest to win a match in ATP Masters 1000 history. 2021: 63rd Grand Slam appearance, Retirement At the 2021 Hall of Fame Open in Newport, Karlovic won against 24-year-old Bernabé Zapata Miralles. This was only his second match win for 2021, the first being at the 2021 Delray Beach Open against Pablo Andújar. At 42 years old, Karlovic was the oldest player to compete in an ATP Tour match since Thomas Muster, 44, in 2011 in Vienna (l. to Dominic Thiem in R1). In July, Los Cabos Open organisers circulated news of Karlović's retirement at the 2021 US Open. Later, he said he may change his mind and continue playing in 2022 if he got good results. Karlović entered the US Open qualifying draw for possibly his last tournament. He won all three of his qualifying matches to qualify for the US Open, making