England's Lee Westwood slipped to eight shots off the lead after scoring a level-par 70 in his second round at the St Jude Classic in Memphis.
The world number two, who is hunting his first major, is preparing for the US Open in Bethesda on 13 June.
Last year's runner-up Robert Karlsson carded a 65 to establish a commanding tournament lead on nine under.
Ireland's Padraig Harrington, a three-time major champion, is well adrift after an underwhelming two-over 72.
Colt Knost and Keegan Bradley are the closest of a clutch of Americans chasing Karlsson on six under.
The Swede, who has yet to win on the PGA Tour, holed three birdies in the last four holes to ensure sole ownership of the lead at the halfway point.