* Johnson and Grace card 70s to move joint top
* Spieth drops back into a tie for lead after fast start
* Floored Day hits back at Chambers Bay (Adds detail, quotes)
UNIVERSITY PLACE, Washington, June 20 (Reuters) - Jason Day wrote his name into U.S. Open folklore on Saturday, grabbing a share of the third-round lead on a punishing Chambers Bay layout that had literally brought the Australian to his knees a day earlier.
Even though Day's round will be long remembered, there is still another chapter to be written in the 115th U.