Spaniard Urtasun pipped Dutchman Marc De Maar (United Healthcare) and Italian Ivan Basso (Liquigas-Cannondale), with Spain's Samuel Sanchez (Euskaltel-Euskadi) at two seconds on a stage featuring the twin peaks of Merrivale and Coffin Stone in Dartmoor.
Briton Tiernan-Locke attacked from the peloton in the final 10 kilometres of the 170.7km stage from Barnstaple to finish eighth, with main rival Leigh Howard (Orica-GreenEdge) now down in fourth at 1:02 despite taking a three-second sprint bonus - to reduce the gap to the leader to just 10 seconds - early in the stage.
After the first classified climb of the day the peloton split then five riders - Basso, Sanchez, Urtasun, Jerome Coppel (Saur-Sojasun) and stage five winner De Maar - escaped from the lead group, which contained Endura Racing's Tiernan-Locke.
Another five riders joined them a minute ahead of the peloton until the final 15km, where the top four finishers broke away from the other six at the front.
Australian Nathan Haas of Garmin-Sharp is 18 seconds off the lead with Italian Damiano Caruso of Liquigas-Cannondale another five seconds back.
There is one stage remaining, a 147.7km run through Surrey from Reigate to Guildford which omits the Olympic road race landmark Box Hill and features four climbs.