Jordan Spieth in charge after amazing Grace shoots 62

American Spieth fired a flawless 65 to finish on 11-under 199, three clear of his compatriot Matt Kuchar (66) while South African Grace conjured up a majestic 62 to achieve what no player had done in 441 previous majors.

There have been 31 rounds of 63 in the sport's biggest tournaments and Grace's feat prompted a standing ovation at the 18th green as he saluted the packed galleries at Royal Birkdale.

"I honestly had no idea that was something historic," the world number 35 told reporters.

"I knew I was playing well and I was just in the zone. I didn't know 62 was the lowest ever but now it makes it even more special. If my caddie knew he never said anything; good on him!"

Spieth, twice major champion, had taken time out from his warm-up session to watch Grace sink his final putt but the world number three was not distracted.

The 23-year-old calmly collected three birdies on the front nine and with Kuchar keeping pace, the pair kept the rest of the field at arm's length.

Kuchar wobbled with a double-bogey at the 16th before fighting back to birdie the 17th on the way to a 66 and Spieth drained a 12-foot putt at the last to give himself a cushion as he bids for a first British Open title.

"It was a really solid round and I'm very pleased with the position I'm in," Spieth told reporters.

"Mentally, it was quite tough today knowing the conditions were good for scoring. But I made a nice start with the birdie at three and I didn't have too much stress in the round."

American Brooks Koepka, the U.S. Open champion, went round in 68 to finish at five under, level with 20-year-old Canadian Austin Connelly who continued his remarkable run at his first major with a 66.

World number two Hideki Matsuyama of Japan carded 66 to join Grace at four under, one ahead of American world number one Dustin Johnson (64), defending champion Henrik Stenson (65), Spain's Rafael Cabrera Bello (67) and American Chan Kim (67).

World number four Rory McIlroy made an early charge with three birdies but the Northern Irishman's hopes of a fifth major title faded with a double-bogey at the 10th hole and he ended at two under.

That was level with England's Ian Poulter, who started the day three shots behind Spieth but failed to take advantage of the conditions and made five bogeys in a scratchy 71.