Novak Djokovic\'s dream season could be at risk of ending on a low note after the world number one lost only his fourth match of 2011 when he went down 2-6, 7-6 (7/4), 6-0 on Saturday to Japan\'s Kei Nishikori in the semi-finals of the Swiss Indoors. Djokovic said he had been playing all week with a sore shoulder similar to the injury which forced him to quit the Cincinnati final in August against Andy Murray before the US Open. \"I could barely serve for much of the match, I was in pain,\" said the Serb, who has won ten titles in 2011 but only returned this week after six weeks out with a back injury. \"My shoulder is very bad, we won\'t even talk about the third set. I have a lot of pain in my body from the competition this week. I hope I can be ready for Paris.\" That Masters event starts on Monday, with Djokovic now in serious doubt of fronting up. He said he is likely to travel to the city and hope for the best. \"I don\'t think I\'ll be able to train for the next few days.\" Djokovic said that his gruelling first-round win over Xavier Malisse on Tuesday could have set the tone for the rest of the week. \"It was a shock to the body,\" he said of the opening struggle. \"Competition is different to practise \"I may have forced things too much I was feeling afraid of the shoulder and what might happen to it. My rhythm on my serve has been completely off.\" Nishikori, ranked 32nd and a semi-finalist last month at the Shanghai Masters, will play one of two Swiss, four-time champion Roger Federer or Stanislas Wawrinka, for his second career title after winning at Delray Beach in 2008. Nishikori has never played either Federer or Wawrinka. Djokovic received treatment twice during the match on his shoulder and was barely present in the  third set, which Nishikori swept through. Nishikori, 21, has moved from outside the Top 50 since after the US Open, exploding into form in the autumn in Asia and Europe. The Djokovic match was his third semi-final in four tournaments and he now has beaten four Top 10 players in his career. Djokovic got off to a flying start with a break for 3-1 and a 5-1 lead. But with the Serb serving for the set, Nishikori managed a great escape with a break, with Djokovic promptly calling for the trainer to massage his right shoulder. The top seed scratched through to take the set 6-4, concluding on a break after barely half an hour. Djokovic had trouble consolidating in the second set, going down a break in the fifth game as his Japanese opponent began to find his range. The Serb moved into position with a 5-4 lead and love-30 on Nishikori\'s serve but could not strike a blow. Instead, the challenger held for 5-all and eventually squared the match at a set each after a tiebreaker. Nishikori won the 71-minute marathon with seven unforced errors while Djokovic\'s errors mounted to a massive 18 in the set. It was all Nishikori in the third set. \"I played well from the second set,\" said the Japanese winner, \"I got my rhythm and hit some unbelievable shots. \"After Shanghai (where he beat Jo-Wilfried Tsonga) I\'ve felt different with my game. I\'m playing differently, more solid from the baseline and not making any stupid errors.\"