Twenty-four groups have to return to the Sao Paulo Golf Club on Friday to finish their first rounds as thunderstorms ravaged through the course on Thursday afternoon.
Thomas Levet, Santiago Luna, Erol Simsek, Gustavo Rojas and 1998 U.S. Amateur champion Hank Kuehne are in the clubhouse at 4-under 67 while Andrew Beal is through 12 holes with the same score.
Nyman began his first round at the 10th hole and opened with a birdie at No. 12 after he hit 7-iron to three feet. He added a birdie at the next hole, the par-5 13th, when he holed a 15-footer.
A two-putt birdie at the final par-5 on his front, the 17th, sent Nyman out at 3-under 32.
Nyman posted a birdie at the 153-yard 2nd and then capitalized on the remaining par-5s, the third and eighth, with birdies for his flawless 65.
'It has been a long time since my name was at the top of the leaderboard,' said Nyman, who is only making his second start of the European Tour season. 'Today was just one of those days when the putts go in. I hit a lot of good approaches which you need to do here as you are not going to hole many long putts.'
Jacquelin also went bogey-free for his 66 on Thursday. Five birdies on the round, including a 40-foot bomb at No. 3, tied Jacquelin for second with Rumford.
'I holed some nice putts,' said Jacquelin, who has not finished higher than a tie for 41st in six starts this season. 'I tried to get some rhythm into the shots. My long irons were good and I managed to control the punch shots well.'
Rumford opened with a bogey but battled back with six birdies, including four on his second nine, for his 66.
'I got off to a fairly shaky start,' Rumford said. 'It wasn't the nerves, I just couldn't get the ball on the fairway. With the fairways so narrow and the greens so small it emphasizes that.'
Six golfers are finished for the day at 3-under, including Argentina's Jose Coceres. Three additional players are 3-under and will have to return to the course for the completion of their rounds.
Click here for full-field scores from the Sao Paulo Brazilian Open