OMAHA, Neb. (AP) - Michael Allen is threatening to run away with the U.S. Senior Open. He fired a 7-under 63 to get to 10 under par -- good for a five-shot lead over Rocco Mediate That's the biggest lead in the 34-year history of the U.S. Senior Open.

Allen was among seven players who shared the first-round lead at 3-under. He birdied five of the first eight holes and eagled the par-5 14th while shooting his best round of the year in hot, breezy conditions at the par-70 Omaha Country Club.

His round of 63 was one off the U.S. Senior Open record of 62 by Loren Roberts at Prairie Dunes in 2006.

Coverage of the U.S. Senior Open begins at 3-pm EST on NBC and WGRZ-TV.