Grace's round, the lowest in the history of the majors, saw him move from four over par overnight to four-under.

Meanwhile, world number one Dustin Johnson made excellent progress in the benign conditions that were in stark contrast to the wet and miserable weather of Friday afternoon.

The languid American shot a bogey-free round of 64, six-under, to climb to three under par overall for the tournament.

Jordan Spieth led at the halfway stage at six-under and he quickly improved to -7 for the championship with a birdie at the 3rd.

Spieth's playing partner, fellow American Matt Kuchar, had a more eventful start with two birdies and a bogey in his first four holes leaving him five-under.

US Open champion Brooks Koepka also went to five-under thanks to a run of three straight birdies from the 3rd.