Harvard and Princeton universities reached the semifinals of the 1 1/2-mile Ladies Plate at the Henley Royal Regatta today, raising prospects of an all-American final on the river Thames Sunday.

The U.S. crews, seeded in opposite sides of the draw, both were involved in close finishes in their quarterfinals. Harvard beat Reading University by one length and Princeton defeated University College, Dublin, by three-quarters of a length.

Harvard has not won the trophy for 10 years, but U.S. crews have won three of the last four.

Princeton's success was achieved against a crew that averaged 25 pounds a man heavier, and was made possible by a brilliant start.

Even when the Irish crew tried to attack in the middle of the race, Princeton held it off and finished in buoyant mood.

"We may be lightweights but we are extremely fit and can certainly win two races in one day tomorrow--the semifinal and final--to win the event," said captain John Andrews. He quickly challenged Harvard by adding, "Actually, I think Isis, our opponents in the semifinals, will be harder than Harvard in the final."

Harvard stroke Amos Gelb, an Englishman, pointing out that his crew has been conserving energy, said, "Our race today was just like every one so far. We got in front . . . and although we did not win by much, that was because we were able to coast home comfortably."