His height of 211 cm (6 ft 11 in) makes him the joint tallest ranked tennis player in history, along with Reilly Opelka.

[18] Although not turning pro until 2000 at the age of 21, Karlović played his first ITF futures event in June 1998 and made two finals in August 1998 and September 1999, losing both.

Karlović debuted for the Croatian Davis Cup team in 2000 against Ireland, where he won the dead rubber.

Karlović caused a major upset at Wimbledon when he beat defending champion Lleyton Hewitt in his first match in a Grand Slam tournament.

In 2005, Karlović reached his first ATP final at the Queen's Club Championships, defeating Lleyton Hewitt and Thomas Johansson along the way.

At age 28, he won his first ATP singles title at the 2007 U.S. Men's Clay Court Championships in Houston, Texas, defeating Mariano Zabaleta in the final.

On 23 June 2007, he won his second title of the year and his career at Nottingham, the last grass tune-up before Wimbledon, defeating seventh seed Stanislas Wawrinka, Gilles Simon, Juan Martín del Potro, second seed Dmitry Tursunov (a match played on indoor hard courts due to torrential rain), and Arnaud Clément.

Karlović edged past his rival after saving a match point in the final-set tiebreak, and then went on to defeat German No.

In the 2009 Davis Cup semifinals, Karlović was defeated by Radek Štěpánek, despite hitting a record-breaking 78 aces (the record has since been taken by John Isner).

[24] Karlović also won his first five-set match in the Davis Cup quarterfinals against the United States, where he came from behind to beat James Blake from two sets down.

As the second seed at the 2010 Delray Beach International Tennis Championships, he made a runner-up effort, losing to first-time ATP-finalist Ernests Gulbis.

In the 2010 Davis Cup, where Croatia had a first-round clash with Ecuador, Karlović defeated Nicolás Lapentti in five sets.

[9] After a lengthy recovery, he was able to play again in Newport in July, where he made it to the quarterfinals, before being defeated by John Isner in one of their typical ace-fests.

At the US Open, he made it through qualifying and into the second round by defeating James Blake in the last singles match of his career in five sets.

In February, Karlović reached the final of the U.S. National Indoor Tennis Championships, losing to Kei Nishikori.

In July, he reached the final of the Hall of Fame Open, losing an epic match to Lleyton Hewitt.

Another solid week of tennis saw him make back-to-back finals, this time at the Claro Open Colombia, losing to Bernard Tomic in a tight three-setter.

He reached an ATP World Tour 500 series tournament at the Washington Open (tennis), which he lost to Gaël Monfils.

He then won the Los Cabos Open, beating Feliciano López in straight sets and returned to the top 20 again at 37 years and 6 months.

At the Ecuador Open in Quito, Karlović made it to the Round of 16, losing to 18 years old Corentin Moutet in a third-set tie-break.

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.

[31] 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.

[32] 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.

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).

Besides his huge serve, Karlović also has a powerful forehand which he usually hits deep and fast into his opponent's baseline.

Only once in his ATP career has he failed to hit an ace during a match, which was in the 2008 Monte Carlo Masters against Frenchman Gaël Monfils.

Karlović's fastest official serve is 156 miles per hour (251 km/h), which he hit during his five-set loss in the 2011 Davis Cup first round third rubber doubles match with Ivan Dodig against Christopher Kas and Philipp Petzschner of Germany.

[9] On 19 June 2015 Karlović broke the record for most aces in a three-set ATP Tour match during the quarterfinal of 2015 Gerry Weber Open.

[56][57] This record was surpassed by John Isner in June 2022, who hit his 13,729th ace in a third round match at the 2022 Wimbledon Championships.

Karlović has previously worn apparel by Diadora, My OCK, Adidas, Nike, Li-Ning, Sergio Tacchini and Mizuno, as well as his own brand 6'10".