Thanks. If you go to the credits, all the music is listed there (at the very end), but the theme you are referring to would be the main theme from the movie 2046 (directed by Kar Wai Wong,
http://www.imdb.com/title/tt0212712/). I can't say if a similar theme also appears in The Professional or not, but if Wong's film score was based on a classical piece (the style of the film could suggest that), it may very well be. Maybe someone else can chime in.