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.