Romero's score tied for the fourth-best opening round in tournament history, and it represented the latest step in a banner summer for Argentine golf. It came less than a month after Romero's countryman and friend, Angel Cabrera, became the first player from Argentina to win the U.S. Open.
Romero held a three-shot lead over another Argentine, Vicente Fernandez, and a pair of club professionals, Jon Fiedler of Camarillo, Calif. and Ron Vlosich of Lakewood, Colo.
Gil Morgan, Loren Roberts, Bruce Vaughan and Jim Woodward, were on the course at 3 under when play was halted because of threatening weather at 5:05 p.m. CDT. Amid thunderstorms and high winds, play was called for the day at 6:15 p.m.
The 78 players who were unable to finish the round will resume at 7 a.m. Friday.
Romero teed off from the 10th hole on Thursday morning, using accurate iron shots to set up back-to-back birdies on 12 and 13. He later rolled in an eagle putt from 50 feet on the par-5 16th hole to go four under par and grab the early lead.
Romero made three more birdies on his second nine, with his only bogey of the round coming on the seventh hole.
Romero is the third player to shoot 6 under in the first round of a U.S. Senior Open. Three have shot 7 under in the first round of a senior open, most recently Craig Stadler in 2005.
Romero was rookie of the year on the Champions Tour last year and won the Jeld-Wen Tradition, which is considered a senior major, last August. He has a close bond with Cabrera that began when Romero's father, a club pro in Argentina, recognized Cabrera's potential to play professionally when he was a 10-year-old caddie.
Romero later spent his own money to sponsor Cabrera on the European Tour.
That investment paid off last month when Cabrera won the 2007 U.S. Open at Oakmont. It was only the second victory in a major championship by an Argentinian player, and the first since Roberto De Vicenzo won the 1967 British Open.
De Vicenzo, a hero to Romero, also won the 1980 U.S. Senior Open.
Whistling Straits was expected to provide a tough test for the over-50 crowd after high winds -- the signature of the relatively tree-free Scottish-style course perched on the banks of Lake Michigan about an hour north of Milwaukee -- blew hard during practice rounds early in the week. But before the storms arrived on Thursday evening, relatively calm winds led to unexpectedly low scores in the first round.
Not everybody came through unscathed.
Two-time U.S. Senior Open champion Hale Irwin struggled through his first nine holes, triple-bogeying the par-four 15th after needing two shots to get out of a greenside sand trap and three-putting.
Jay Haas, regarded as a tournament favorite after winning four times on the Champions Tour earlier this season, shot a 1-over 73 with nagging soreness in his back.
Allen Doyle, trying to become the first player to win three straight U.S. Senior Opens, was 7 over after 13 holes when play was called.