John Isner retires from Australian Open

American number one John Isner is out of the 2014 Australian Open after retiring hurt from his first-round clash with Slovakia's Martin Klizan.

The 13th seed, who won the Heineken Open in Auckland on Saturday, lost the first two sets to Klizan before quitting due to a knee injury.

Isner's usually dependable serve misfired in the opening set as he was broken twice en route to going down 6-2.

He then saved a single break point in game three of the second, which went all the way to a tie-break.

Isner raced into a 4-0 lead in the tie-breaker. However, Klizan fought back to level at 4-4 before moving 6-5 ahead.

The first set point was saved by Isner's 10th ace of the match, but Klizan was not deterred and won the next two points to take the tie-break 8-6.

Just two points were played in the third set before Isner called it a day. Both were won by Klizan, who will face either Blaz Rola or Federico Delbonis in round two.