It's the maximum on-site fine a Grand Slam tennis tournament can issue for unsportsmanlike conduct.
With Williams serving, two points from defeat, a line judge called a foot fault on a second serve. That gave Clijsters a match point. Williams screamed, cursed and shook a ball in the official's direction. Williams was docked a point for unsportsmanlike conduct, ending the match.
The U.S. Open said Sunday the Grand Slam Committee Administrator has opened an investigation to determine whether Williams' behavior should be considered a "major offense," which can lead to additional penalties.
Williams also is being docked $500 for racket abuse in what became a 6-4, 7-5 loss Saturday night.
Williams earned $350,000 in prize money for reaching the singles semifinals. She also is scheduled to play in the women's doubles final Monday with her sister Venus.