The Potomac Boat Club of Washington, D.C., the lone U.S. survivor in the Thames Challenge Cup for light-weight heights, gained the semifinals in the Henley Royal Regatta yesterday.

The Washington-area weekend oarsmen, whose ages go up to 43, defeated the Northeastern University freshmen by 2 1/3 lengths in 6:57. They will race the seeded Leander Club today and the final will be later in the day on the River Thames.

University of Washington crew Coach Dick Erickson received a bottle of vodka from the coach fo the Bulgarian national crew his Huskies face today in the final Regatta's top event, the Grand Challenge Cup.

Washington, undefeated at home and winners of the Grand here last year, say they are better crew than in 1977 when they beat the British national squad to take the heavyweight eight title.

But the Bulgarians, racing at Henley for the first time, dismissed the British national eight on Friday and rowed past Syracuse University varsity in yesterday's semifinals.

Washington coasted to a 3 3/4-length win over Northeastern University in the other semifinal.

Erickson said, "The question is can we pull out our best effort in one 6 1/2 minute boat race Sunday. The Bulgarians will just take off and keep the pressure on all the way."

Bulgarian Coach Nikolai Zdravkov was not saying much. Asked if his crew can beat the Americans, he shrugged his shoulders and smiled.

Fourteen U.S. eights came to Henley this year, but going into the final day only four remained.In addition to Washington and Potomac, Yale University's freshmen and the Syracuse freshmen-junior varsity crew and quarter final victories in the Ladies' Challenge Plate.

Yale beat its Connecticut neighbors, Trinity College, Hartford, by 1 1/2 lengths, although the Eli at one point seemed intent on sinking Trinity by steering straight at them. Trinity obligingly altered course to avert the collision and plowed into buoys on the water's edge. No one claimed, however, that the incident determined the final outcome.

The Syracuse junior crew, in the same event, had a comfortable 1 2/4 length win over Isis Boao Club of England. Rick Ritter Jr., captain of the 1977 Yorktown High (Arlington, Va.) crew, rows No. 5 for Syracuse.

In the Diamond Challenge Sculls, which has a world-class field, 1975 world champion Peter Kolbe of West Germany lost in the semifinal to new-comer Hugh Matheson of England.