Vuelta a Espaņa - Cancellara to quit Vuelta
"This Vuelta has been really demanding. Besides the time trial in Salamanca, I was dedicated to help Daniele Bennati in the sprints and Jakob Fuglsang and Maxime Montfort, our duo for the GC, in the climbs," said Cancellara.
"In order to be fresh in Copenhagen, I need to take some rest now and then pick up training again."
The Swiss powerhouse is the reigning time trial world champion and will be amongst the favourites to retain his title on Wednesday 21 September.
Last year in Melbourne, 30-year-old Cancellara became the first man to win four world time trials, following previous victories in 2006, 2007 and 2009.
Olympic time trial champion Cancellara, who will also ride in the road race on Sunday 25 September, said he was looking forward to defending his crown.
"I think a reigning world champion should always defend his jersey, so I'm going to try to win the time trial for the fifth time.
"I'm very pleased that the time trial is already on Wednesday, which allows for an extra day of recuperation in view of the road race on Sunday."
Cancellara finished fourth in the Vuelta's 47km time trial in Salamanca last week, one minute and 27 seconds behind winner Tony Martin of Germany.
With victories in both the Vuelta and Tour de France ITT over Cancellara this season, Martin will perhaps be installed as the favourite for the rainbow jersey in Copenhagen.
After the second rest day in the Vuelta, Cancellara was 136th in the GC, almost three hours down on race leader Juan Jose Cobo of Spain.