Towering Croat Ivo Karlovic announced his retirement from tennis at the age of 44 on Wednesday, bringing the curtain down on a career that saw him fire more than 13,000 aces past bewildered opponents.

Standing 2.11m tall, Karlovic's huge serves and unorthodox style caused problems for some of the sport's biggest names and he won two of his three meetings with world number one Novak Djokovic.

Having last played on tour at Indian Wells in 2021, and with his 45th birthday approaching this month, Karlovic confirmed his retirement to fans on social media.

"Some of you have noticed that it's been a while since I played a tournament and still ask me on a daily basis if and when I'll be returning to the tour," he wrote.

"I'm sorry to disappoint, but with this letter, I want to make it official and finally announce my retirement."

Karlovic turned professional in 2000 and made a splash in his first Grand Slam match three years later by beating defending champion Lleyton Hewitt at Wimbledon.

His best result at a Grand Slam also came at Wimbledon in 2009 when he reached the quarter-finals.

Karlovic will also be remembered by fans for his longevity in a physically demanding sport, while his 13,728 aces stands second only to American John Isner's tally of 14,470.

He also produced one of the fastest serves in the game - a 251kph blast in a Davis Cup match in 2011.

Karlovic achieved a career-high ranking of number 14 in 2008 after getting past then world number one Roger Federer en route to the Cincinnati semi-finals.

"I've had a very gratifying, unorthodox and long career coming from extremely humble beginnings, especially for a tennis player," Karlovic added.

"Croatia was a very different country in the 80s and 90s when I was growing up, and I'm very proud of my accomplishments and what I've been able to overcome."

Karlovic finishes his career with eight titles to his name, the last of which came in 2016. REUTERS