MADRID -- After a seven-month hiatus nursing a hurt knee, Rafael Nadal is wary about his upcoming return to the tennis court and believes it may be some time before he is back in top form."I have my doubts. It's normal. We are talking about a knee, so of course I am afraid to see how it is going respond," Nadal told Canal Plus television Friday. "But I can only trust my doctors and believe in myself and that everything will be all right."The 26-year-old Spaniard is set to play an exhibition tournament in Abu Dhabi on Dec. 27. It will be his first action since he was sidelined with tendinitis in his left knee after a second-round loss to 100th-ranked Lukas Rosol at Wimbledon in June.The injury prevented Nadal from defending his Olympic singles gold at the London Games, where he was supposed to be Spain's flag bearer in the opening ceremony. He also had to pull out of the U.S. Open and Spain's Davis Cup final against the Czech Republic, which his teammates lost without him.The 11-time Grand Slam winner and former No. 1 player said his knee had improved over the last two months after making frustratingly little progress during the summer.Even so, he acknowledged that he may have to skip some more events to get back to full speed."I'm prepared to accept that at the start my knee might not respond well and I may have to take it easy, mixing periods of play and rest for the first three months," he said.Nadal said that he wanted to play at Indian Wells and Miami with the goal of being completely fit by April to play at Monte Carlo, a clay-court tournament he has dominated for eight consecutive years.The Abu Dhabi tournament features a six-man field that includes top-ranked Novak Djokovic and No. 3 Andy Murray of Britain.