Sergio Garcia leads the WGC-American Express Championship in Atlanta by one stroke after the first round.
Garcia shot a five-under 65 to edge ahead of the American pair of Tim Herron and Rocco Mediate.
Tiger Woods put his new driver to good use as he opened with a 67 to move alongside South Korea's KJ Choi.
Ernie Els shot 71, while Phil Mickelson's challenge already looks to be over after a disappointing 73.
Garcia was delighted that the hard work he has been putting in on his swing is starting to pay off.
"I've just got to keep working hard and wait for the moment - it doesn't come overnight," said Garcia, who decided to make swing changes at the start of the year.
"It's difficult to stay patient, but it feels a lot better now."
Woods has also been making changes, but in his case to his equipment.
He is playing with a new Nike driver and is happy with the results.
"I hit the driver great and it was nice to see a high draw back. I'm five to seven yards longer with it as well," he said.
"Because I've not played for a month I was nervous on the first tee and the course is playing very difficult.
FIRST ROUND (US unless stated)
-5 Sergio Garcia (Spa)
-4 Tim Herron, Rocco Mediate
-3 KJ Choi (S Kor), Tiger Woods
"The fairways are a bit on the narrow side and the rough is up."
Northern Ireland's Darren Clarke was happy with a round of 69 after making a 20-footer for birdie on the 17th, and getting up and down from sand at the last.
"I had a lot of opportunities and wasted two or three shots, but it's tough and 69 is OK," he said.
"I'm getting to like demanding courses more because I'm doing better at the challenge."