Adam Scott and Jason Dufner both hit rounds of 66 to share the lead after day one of the WGC-Cadillac Championship in Miami, Florida.
Scott led for most of the day, but American Dufner eventually joined the Australian on six-under in windy conditions.
Denmark's Thomas Bjorn and South Africa's Charl Schwartzel both carded rounds of 68 to finish on four-under.
Britain's highest-placed competitor was Justin Rose, who took a share of third with Steve Stricker on three-under.
Rory McIlroy endured a tough opening round to the event as the newly-crowned world number one ended the day with a one-over 73.
Elsewhere, Lee Westwood carded a 76 and Luke Donald posted a round of 69.
