Sweden's Carl Pettersson and Argentina's Angel Cabrera set the clubhouse target before bad weather brought an early end to play in the first round of the Shell Houston Open.
Cabrera birdied the 18th to complete a flawless seven under par 65 shortly before lightning in the area forced the players from the course at Redstone Golf Club.
Pettersson was already safely in the clubhouse by that point after carding eight birdies and one bogey - on the 17th - in his opening round.
Pettersson, out of form coming into the tournament, admitted even he had been slightly surprised by his round.
Asked where it had come from, he said: "I don't know. I've missed last two cuts. I haven't played very well and changed my set-up a little bit and it seems to be working.
"I started off great with some birdies and it was a fun round. I really enjoyed it. I hit it well and played some putts."
American duo Jeff Maggert and Ricky Barnes were a shot further back after rounds of 66, while England's Lee Westwood also made a good start with a round of 68 to lie four under.
Westwood will make his 56th attempt to win a Major at next week's Masters Tournament, and for the fourth year running is warming up in Houston.
“It was good to be able to work through it and post a decent score,” said Westwood.
“Now the idea over the next three rounds is to continue to get sharper for Augusta and hopefully get myself in contention at the weekend.'
Westwood was runner-up to Mickelson at Augusta two years ago, while Cabrera claimed the coveted Green Jacket in 2009.
One shot behind Westwood is Dane Thomas Björn, who carded five birdies and would have been higher than tied for 16th but for a double bogey six on the 11th.
By the time the rain came, 51 players had finished their rounds, and organisers scheduled a start time of 7.30am (13.30 BST) for Friday.
Westwood’s playing partner Grameme McDowell continued his good form with a round of 70, having finished runner-up to Tiger Woods last week.
“It's been a while since I've headed to Augusta with the nice, high ball flight with my irons that you need at the Masters,” said McDowell.
