Mickelson bewildered by 'adjustment penalty'

U.S. team member Phil Mickelson watches his tee shot on the second hole during the four ball matches of the 2015 Presidents Cup golf tournament in Incheon, South Korea, October 9, 2015. REUTERS/Kim Hong-Ji

INCHEON, South Korea (Reuters) - An obscure penalty clause, tucked away in the farthest depths of golf's rulebook, took centre stage at the Presidents Cup on Friday after Phil Mickelson changed the type of ball he had been playing with mid-round.

Mickelson and U.S. playing partner Zach Johnson were all square through six against the Internationals' Jason Day and Adam Scott when Mickelson switched to a harder ball to get extra distance from the tee going into the par-five seventh.