Premise: The 2009–10 Tour de Ski was the 4th edition of the Tour de Ski and took place 1–10 January 2010. The race started in Oberhof, Germany, and ended in Val di Fiemme, Italy. The defending champions are Switzerland's Dario Cologna for the men and Finland's Virpi Kuitunen. This year's event was won by Lukáš Bauer of the Czech Republic for the men and Poland's Justyna Kowalczyk for the women.
Hypothesis: The  5th edition of the 2009–10 Tour de Ski took place 1–10 January 2010.