The Bora-Hansgrohe rider's sprint finish was enough to leave UAE Team Emeriates' Fernando Gaviria in second place.
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Fabio Fabio Jakobsen (Deceuninck-QuickStep) finished third.
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Britain's Team Ineos kicked off the sprint at the Aleja Focha in Krakow, but he was unable to sustain his effort and soon dropped out from the front. That sparked the rest of the peloton into action, and Gaviria - returning from a knee injury- began an attack o his own.
That was not enough to unsettle Ackermann, who won the same stage at last year's championship, and he raced by Gavirira with a little over 100 meters remaining in the race.
Mark Cavendish (Dimension Data) , who is making his return from Epstein Barr virus, had little more luck when he was taken out by a crash on the last lap of the finishing circuit. The 34-year-old Briton was nonetheless able to complete the tage.
1) Ackermann Pascal (BORA - hansgrohe)
2) Gaviria Fernando (UAE-Team Emirates)
3) Jakobsen Fabio (Deceuninck - Quick Step)
4) Walscheid Max (Team Sunweb)
5) Van Poppel Danny (Jumbo-Visma
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