By Richard Martin BARCELONA, Sept 8 (Reuters) - Dane Magnus Cort Nielsen edged his way to victory in stage 18 of the Tour of Spain on Thursday, leading out the sprint in the final 500 metres of the 200.6km race and taking the stage win in his first appearance in a Grand Tour. The peloton kept close to the breakaway group in the second longest stage of the Tour, from Requena to Gandia, so leader Nairo Quintana maintained his 3:37 lead over nearest challenger Chris Froome before Friday's individual time trial. Friday's 37km time trial represents Froome's best chance of making up time on favourite Quintana, although his hopes of wresting the red jersey from the Colombian have diminished since he finished 2:37 behind him in stage 15. More...