On 18 September 2009, Karlović hit 78 aces (77 aces on 1st serve) in a Davis Cup match against Radek Štěpánek, breaking his previous record of 51 (against Daniele Bracciali in the 2005 Wimbledon Championships) and 55 (against Lleyton Hewitt in the 2009 French Open).
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.
His record of aces in a single match was broken by both John Isner and Nicolas Mahut in the longest match in tennis history at the first round of the 2010 Wimbledon Championships, where Isner hit 113 aces and Mahut hit 103.

In 2007, Karlović became the fourth player to serve 1,000 aces in a season.
He finished the year with 1,318 aces, second all-time behind Goran Ivanišević, who hit 1,477 in 1996.
In 2015, he became the third player (after Goran Ivanišević and Andy Roddick) to serve 9,000 career aces.,
and the second to notch 10,000 aces.

Karlović's fastest official serve is 156 miles per hour (251 km/h), which he hit during his five-set loss in a Davis Cup first round third rubber doubles match with Ivan Dodig against Christopher Kas and Philipp Petzschner of Germany.
That was ATP's official record at the time.
His previous record was 153 miles per hour (246 km/h) in a victory over Arnaud Clément in the final of Nottingham in 2007.
This is the third-fastest first serve on record, after Andy Roddick's 155 miles per hour (249 km/h).

He also hit a 144 miles per hour (232 km/h) second serve against Paul Capdeville in the quarterfinals of the 2007 Legg Mason Tennis Classic on 3 August, which is the fastest second serve on record.

Karlovic's record of a 156 mph serve was broken by Samuel Groth at the 2012 ATP Challenger in Busan, but was still the record for ATP events until John Isner broke the record in 2016.

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.
He served 45 aces in a 7–5, 6–7, 6–3 win over Tomáš Berdych.

Karlović holds the record for the most aces in tennis history after surpassing Goran Ivanišević in October, 2015.
As of February 2020, he has struck 13,588 aces in 686 matches.
On 18 September 2009, Karlović hit 78 aces (77 aces on 1st serve) in a Davis Cup match against Radek Štěpánek, breaking his previous record of 51 (against Daniele Bracciali in the 2005 Wimbledon Championships) and 55 (against Lleyton Hewitt in the 2009 French Open). 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. His record of aces in a single match was broken by both John Isner and Nicolas Mahut in the longest match in tennis history at the first round of the 2010 Wimbledon Championships, where Isner hit 113 aces and Mahut hit 103.
In 2007, Karlović became the fourth player to serve 1,000 aces in a season. He finished the year with 1,318 aces, second all-time behind Goran Ivanišević, who hit 1,477 in 1996. In 2015, he became the third player (after Goran Ivanišević and Andy Roddick) to serve 9,000 career aces., and the second to notch 10,000 aces.
Karlović's fastest official serve is 156 miles per hour (251 km/h), which he hit during his five-set loss in a Davis Cup first round third rubber doubles match with Ivan Dodig against Christopher Kas and Philipp Petzschner of Germany. That was ATP's official record at the time. His previous record was 153 miles per hour (246 km/h) in a victory over Arnaud Clément in the final of Nottingham in 2007. This is the third-fastest first serve on record, after Andy Roddick's 155 miles per hour (249 km/h).
He also hit a 144 miles per hour (232 km/h) second serve against Paul Capdeville in the quarterfinals of the 2007 Legg Mason Tennis Classic on 3 August, which is the fastest second serve on record.
Karlovic's record of a 156 mph serve was broken by Samuel Groth at the 2012 ATP Challenger in Busan, but was still the record for ATP events until John Isner broke the record in 2016.
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. He served 45 aces in a 7–5, 6–7, 6–3 win over Tomáš Berdych.
Karlović holds the record for the most aces in tennis history after surpassing Goran Ivanišević in October, 2015. As of February 2020, he has struck 13,588 aces in 686 matches.
Ivo Karlović