- World No. 1 Novak Djokovic beaten by Kei Nishikori in Swiss semifinals
- Serbian in doubt for next week's Paris Masters due to shoulder problem
- Japan's Nishikori plays defending champion Roger Federer in Sunday's final
- Juan Martin del Potro's Valencia defeat puts Tomas Berdych in ATP World Tour Finals
Novak Djokovic has had a phenomenal 2011, but his exertions appear to be taking their toll. Injuries are threatening to derail the world No. 1's hopes of finishing the tennis season on a high note.
Djokovic suffered just his fourth defeat in 71 matches this year on Saturday, losing 2-6 7-6 (7-4) 6-0 to Japan's rising star Kei Nishikori in the semifinals of the Swiss Indoors event.
The Serbian has won three out of four grand slam titles this season, but is battling to be fit for the season-ending ATP World Tour Finals after a recurring shoulder problem hampered him in Basel.
Djokovic, the champion here in 2009 and runner-up last year, said he would travel to France for next week's penultimate event, the Paris Masters, despite his fitness worries.
"I don't think I'll be able to train for the next few days. I could barely serve for much of the match, I was in pain," the 24-year-old told reporters after losing a set 6-0 for just the sixth time in his career.
"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."
Djokovic returned to action in Switzerland after six weeks out with a back injury suffered during his epic U.S. Open final against Rafael Nadal, an ailment which left him unable to guide Serbia into a second successive Davis Cup final.
"It was a shock to the body. Competition is different to practice. I may have forced things too much," he said.
"I was feeling afraid of the shoulder and what might happen to it. My rhythm on my serve has been completely off."
World No. 32 Nishikori made history by becoming the first Japanese player to beat a top-ranked opponent.
The 21-year-old, a wild-card entry after reaching the Shanghai Masters semis, will play defending champion Roger Federer in Sunday's final as he seeks his second ATP Tour title -- the first came in 2008.
Third seed Federer has won four of his 67 career titles in his hometown event, where he has now won 42 out of 49 matches.
The 16-time grand slam champion defeated compatriot Stanislas Wawrinka 7-6 (7-5) 6-2 on Saturday -- his 10th win in their 11 meetings -- to reach the final for the sixth year in a row. Apart from his 2009 defeat by Djokovic, Federer also lost in the 2000 and 2001 finals.
Juan Martin del Potro's hopes of reaching the eight-man season finale suffered a blow on Saturday when the Argentine lost in the semifinals of the Valencia Open hard-court event.
The sixth seed could have moved up to ninth in the rankings with victory, and was in good form having not dropped a set all week, but was surprisingly beaten 6-4 7-6 (7-4) by last year's runner-up Marcel Granollers.
The result means that Tomas Berdych became the sixth player to qualify for the event, the second year in a row the Czech has done so.
World No. 34 Granollers reached his fourth career final, having won two titles on clay.
He will next face Argentine wildcard entry Juan Monaco, who upset defending champion David Ferrer -- beating Granollers' fellow Spaniard 7-5 1-6 6-3.
Ferrer has already qualified for the London showpiece later this month.