Mark Talbott, the youngest professional in the field, defeated defending champion Clive Caldwell yesterday and advanced to the semifinals of the $15,000 Wurzburger Open squash tournament at the Washington Squash Racquets Club.

Talbott, 21, of Toronto, won, 9-15, 15-12, 15-9, 16-13. He is also the youngest pro on the World Professional Squash Association tour.

In two seasons on the tour he has earned a reputation as one of the top retrievers in the game. It was this ability, along with unforced errors by Caldwell, that brought Talbott's head-to-head record against him to 4-3.

This season (which began in September), Talbott has advanced to the finals in three of six professional tournaments and has won in Indianapolis and Montreal.

Talbott took up the 150-year-old English game at age 8 while living in Dayton, Ohio. "I lived in an old house that my grandfather built that had a squash court," Talbott said. "We played basketball on it, too."

It was only natural that he and his older brother Dave, 29, who also is on the pro tour, took up the sport that their father Douglas also plays.

Dave Talbott of Detroit lost in yesterday's second round. Their father, a physician in Atlanta, also was scheduled to play in the amateur over-50 competition but had to cancel because of a heavy work schedule.