Adam Scott and Johnson Wagner were tied for the lead at nine under par after the first round of the Houston Open.
Australian Scott fought off the effects of virus to equal a course-record 63, previously established by Wagner and Bubba Watson last year.
Wagner then went out and matched Scott's score.
Charlie Hoffman was two strokes behind, while Steve Stricker was three strokes off the lead.
Scott's round opened with a 55-foot birdie putt. He has now has played 54 holes without a bogey at this event.
Johnson's round included an eagle on the eighth, where his approach shot from 262 yards came to a rest a foot from the pin.
Phil Mickelson, the highest ranked player in the field, shot an even-par 72 - while Ireland's Padraig Harrington was one under.
Jose Maria Olazabal and Jesper Parnevik both finished with three-under 69s.