Tennis star Rafael Nadal had to retire during his match against Briton Andy Murray in the Australian Open as his knee problems flared up again.

Nadal was already trailing by two sets and a break when he decided to stop playing in the third set.

The Spaniard, who suffered persistent tendonitis problems last year, said he made the decision as he knew he was going to lose anyway and that continuing might aggravate the injury.

Explaining his decision afterwards, the 2009 champion said: "So I said, well, no repeat the same mistake like I had last year. I go to the limit, but not cross the limit, no?"

The persistence of his knee problems may raise doubts over the 23-year-old's long-term future in top level tennis.

People struggling with tendonitis in the knee may find that ultrasound treatment helps aid the problem.

According to Physioroom.com, the most common injury tennis players will suffer is a sprained ankle, with shoulder pain the second most prevalent.