Spain\'s former tennis world number one Rafael Nadal, who has struggled with a sore left shoulder since the end of last season, said on Sunday he is not sure he will be playing at his peak at the Australian Open later this month. \"I know I am working well, with a much more positive attitude than I had during the second half of last season, especially in the final stretch,\" he said in an interview published on Sunday in Catalan sports daily Sport. \"I see myself capable of having a good tournament but you never know. I am happy with the level of my game, of how I am doing things and I think I am sowing seeds that will bear fruit. But maybe not in Australia.\" The Australian Open, the season\'s first Grand Slam tournament, gets underway on January 16. Nadal, the world number two, slumped to a 6-3, 6-4 loss to France\'s Gael Monfils, the world number 16, in the Qatar Open semi-finals on Friday. The 25-year-old winner of the 2009 Australian Open said he suffered from greater mental fatigue than usual last year. \"Mental, and in some moments also physical fatigue, but mainly mental,\" said Nadal, who nevertheless found the reserves to lead his compatriots to Davis Cup glory over Argentina at the end of the campaign. \"In some moments the season was a bit heavy for me. It is many years of a routine, of maximum intensity every week. \"When a new season starts, everything is different. At least you have to do everything in your reach to make it so.\"