Dortmund thump Mainz to close in on Bayern
Wing-back Ryerson laid on four Dortmund headers, including Guirassy's brace and one each from Maximilian Beier and Ramy Bensebaini in Dortmund's biggest league win this season.
Bayern, who play at struggling Werder Bremen on Saturday, were 11 points clear of Dortmund at Christmas having played the best opening half to a season in Bundesliga history.
A six-match winning run has let Dortmund cut into their lead. The two face off in Dortmund in two weeks in a match with huge title ramifications.