The Swedish 17 year-old enhanced his growing reputation with a fine display that began by cancelling out Erfurt's shock early opener, before bagging a second-half hat-trick to sew up a hard-earned victory.

The friendly came less than 24 hours after Dortmund's penalty shootout defeat to Bayern Munich and Peter Bosz started only three players - Mahmoud Dahoud, Felix Passlack and Maximilian Philipp - from that match.

Dortmund's disappointment deepened when Lion Lauberbach handed the third-tier host a 12th-minute lead, but Isak and Jan-Niklas Beste quickly overturned the deficit.

Isak - who has been dubbed the 'new Zlatan Ibrahimovic' - added two more classy finishes after Elias Huth's equaliser and capped his haul with a fourth just before full-time.

The Bundesliga side now looks ahead to its opening fixture away to Wolfsburg on 19 August as it aims to improve on last season's third-place finish.