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 hosts 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 look ahead to their opening fixture away to Wolfsburg on 19 August as they aim to improve on last season’s third-place finish.