Rafa's injury should not end his career
Tendinitis means inflammation of a tendon. In tennis players whose knees are affected, the commonest type is patellar tendonitis, caused by repetitive overuse.
The condition causes pain at the front of the knee just below the patella (kneecap) and is worst when the knee is flexed, as when bending for a low shot.
Professor Angus Wallace, former chair of the National Institute of Sports Medicine said it could be difficult to predict how long Rafael Nadal might take to recover.
"It will be aggravated by continuing to play. If it is aggravated badly it will set him back for other tournaments. The injury is usually related to over-use combined with small tears of the tendon where it is attached to the bone."
Professor Wallace said the injury would need treatment. Physiotherapy, involving transverse rubbing across the tendon, could help. Ultrasound was also sometimes used. In severe cases surgery can be required but is not often used.
The injury was not career-ending and with rest would probably resolve within a fortnight, Professor Wallace said.
In an older, less fit person it could take months to heal.
Cristiano Ronaldo: Unhappy forward is ready to quit Real Madrid, according to Angel Di Maria
Commonwealth Games 2014: Australia launch Glasgow swimwear - but are criticised for drawing attention to the 'crotch' area
Borussia Dortmund vs Arsenal: Jurgen Klopp concerned with Dortmund injuries and praises signings of Danny Welbeck and Alexis Sanchez
Alexis Sanchez: The many (animated) faces of Arsenal's Chilean forward
Cristiano Ronaldo to Manchester United: Ramon Calderon claims Real Madrid star is 'fed up'
- 1 Scottish independence: Ireland since 1919 is a lesson for Scotland in what a Yes vote means
- 2 Thailand deaths: Pair's bloodied bodies found naked on Koh Tao beach
- 3 Lego breaks out of the toy box and heads for the gallery
- 4 Vogue under fire for 'Big Booty' article
- 5 Julian Assange and Edward Snowden join piracy mogul Kim Dotcom’s political campaign in New Zealand
Daniele Watts: Django Unchained actress detained by Los Angeles police after being mistaken for a prostitute
The political class is doing what Hitler couldn’t – destroying Britain
Scottish independence: Nationalist leader Jim Sillars threatens pro-union companies with 'day of reckoning' after independence
Scottish independence: Yes campaign feels the heat as Alex Salmond's NHS claims come under furious attack
£23m Birmingham cycle scheme is attacked by Tory councillor for not catering to the elderly
Salmond accused of laughing off national debt with ‘what are they going to do: invade?’ joke