Padraig Harrington hit his lowest-ever official score to lead after day one of the Transitions Championship in Palm Harbor, Florida.
The Irishman carded a course record 10-under-par 61 to give him a three-shot advantage over the USA's Will Claxton.
American Jason Dufner was amongst a number of players who were tied for third on five-under.
England's Justin Rose, who won last week's WGC-Cadillac Championship in Miami, posted a round of 67 to end the day on four-under with former world number one and fellow countryman Luke Donald.
