France's Louis Barré finished third, 4 seconds behind the winner.
Lemmen finished 11th, 5 seconds behind Christen, but retained the overall lead.
Twenty-one-year-old Polish rider Filip Gruszczyński, competing for the national team, finished 36th, 49 seconds behind Christen.
Michał Kwiatkowski, a former world champion and the second-best Polish finisher, placed 49th on the stage, 1 minute and 31 seconds behind Christen. He dropped from 14th to 31st overall, 1 minute and 59 seconds behind the race leader.
Gruszczyński is 29th overall, 1 minute and 26 seconds behind Lemmen.
In the overall standings, Lemmen leads Italy's Christian Scaroni by 6 seconds, with Brenner third, 15 seconds behind.
The race continues on Saturday with a pivotal 125.5-kilometre stage starting and finishing in the southern mountain resort of Bukowina Tatrzańska and featuring a series of challenging climbs.
The 83rd Tour de Pologne began on Monday in the northern port city of Gdynia and ends on Sunday with a 12.4-kilometre individual time trial in the historic town of Wieliczka.
The seven-stage, 1,100-kilometre race is taking riders from Poland’s Baltic coast in the north to the Tatra Mountains in the south.