By Steve Keating

RIO DE JANEIRO (Reuters) - Triple jumper James Connolly won the United States' first gold medal at the 1896 Summer Games and it is expected that on Sunday or Monday a still unknown American will claim a milestone 1,000th gold to bookend 120 years of Olympic domination.

To underscore the United States' status as the Olympics undisputed superpower, at the current rate of winning medals (going back to 2000) it will take almost a century and 21 more Summer Games for the next nation to reach the 1,000 gold plateau, with China finally hitting the mark in 2100.

The 1,000 medals would also represent almost one-quarter of the total gold awarded at the Summer Olympics with no other country even reaching half that number.

The U.S. arrived in Rio sitting on 977 gold and, according to, the former Soviet Union with 473 ranks second with Germany a distant third with 288.

"I was at the Casa Italia event the other night and they were approaching their 200th gold medal, I think they have one more to go, so it just goes to show the scale of accomplishment our athletes have achieved over the many years," said United States Olympic Committee chairman Larry Probst.

With the U.S. medal machine already humming, piling up gold in the swimming pool and gymnastics, the milestone could go to one of the Rio Games biggest names like Michael Phelps or Simone Biles or it could belong to an unknown competing in men's Greco-Roman wrestling or sailing.

It would seem likely the historic moment will come in the pool or athletics stadium with those two sports accounting for more than half the U.S. gold production.

Phelps, the most decorated Olympian ever, has made the biggest individual contribution, chipping in with 22 golds and counting.

The United States' Olympic domination has not been all-encompassing.

No American has ever stood atop the podium in field hockey, triathlon, badminton, handball, table tennis and modern pentathlon as well as polo and lacrosse, which are no longer on the Olympic program.

The United States has hosted four Summer Games in St. Louis (1904), Los Angeles (1932, 1984) and Atlanta (1996).

The first Games of the Modern Olympics produced the fewest golds (11) while the Soviet-boycotted 1984 Los Angeles Olympics generated the most (83).

"I think what it means is we have just a great sporting culture and a really amazing system of sport in our country," said Alan Ashley, the USOC chief of sport performance. "It is very exciting to think of it in those terms and to realize that we are on the doorstep of that amazing achievement."

(Reporting by Steve keating, editing by Neil Robinson)