Dec 19 (Reuters) - Spain"s Rafa Nadal said he cannot guarantee he will make the trip to Melbourne for next month"s Australian Open after returning to action at the Mubadala World Tennis Championship in Abu Dhabi. The 20-times Grand Slam champion"s comeback at the exhibition event -- his first tournament appearance since the Citi Open in Washington in August -- ended in two close defeats by Andy Murray and Denis Shapovalov. Nadal, 35, spent four months on the sidelines with a foot injury. He was knocked out at Roland Garros in the semi-finals and was forced to pull out of Wimbledon, the Tokyo Olympics and the U.S. Open. "The idea is to go there and try my best there in Australia... Being 100% honest, I can"t guarantee it," Nadal said. "I need to speak with my team. "Its more than six months since the last real official match. Things are difficult and I accept that... I was able to compete in both matches and even had my chances, so if we put that in perspective it"s a very positive thing. "I went through this process unfortunately a lot of times in my tennis career but I need to practice and I need to be healthy enough to go through this process. If that happens, I"m confident that I will be back." World number six Nadal said he would not have returned to tennis if he did not feel he was competitive enough to challenge for the biggest titles in the sport. "I am not playing at all for money or just for fun, I am playing to keep achieving goals or at least to enjoy the process to try to achieve my goals," he added. "If then you don"t achieve, it"s fine, but the motivation and the passion is still there." The Australian Open begins on Jan. 17.