The 11-times Grand Slam singles champion, who has not competed since being stunned by Czech Lukas Rosol in the second round at Wimbledon in June, completed a session under the supervision of coach and uncle Toni Nadal at an indoor facility in his home town of Manacor on the island of Majorca.
Nadal, 26, hopes to be back to full fitness in time for the Australian Open in January.
"I think I am feeling better, the knee is improving the right way and I am happy to be on court another time after a few months outside," Nadal told reporters.
"I am just enjoying the feeling to be playing tennis another time and, well, it will be another period of my recovery.
"I start very slow and I go day by day. I will work hard in my comeback and hopefully the knee will be ready soon.
"Always you have to be careful. A few months outside of competition the body has to re-adapt everything on court and all the muscles need to start very slow."
Nadal's athletic, aggressive playing style places huge demands on his muscles and joints and has been sidelined several times by knee problems during his 11-year career.
His latest injury was diagnosed as a partial tear of the patella tendon and an inflammation of the Hoffa's fat pad in his left knee. After the shock defeat at Wimbledon he was unable to defend his Olympic title at the London Games.
As well as the U.S. Open, which he won in 2010 and where he lost to Novak Djokovic in last year's final, Nadal missed the season-ending Tour Finals and Spain's Davis Cup final defeat to Czech Republic at the weekend.
"Every day is better otherwise I wouldn't be on the court," he said.
"The images say the evolution has been favourable and that is reason for satisfaction after many months of working and waiting.
"It is now another phase where I'm starting little by little and going day by day with patience to find the right moment when all is well and my body will be ready to compete and when my knee is ready."
Nadal's enforced absence deprived tennis's "big four" of seven-times French Open champion, Djokovic, Roger Federer and Andy Murray of one of its most competitive and successful members for the second half of the season.
Nadal had played some of his best tennis in the first six months of 2012, losing narrowly to Djokovic in the final of the Australian Open and winning a record seventh Roland Garros title on his favoured clay.
- Sports & Recreation
- Toni Nadal