The unseeded Indian beat the world number two and All England champion Marin 21-15 18-21 21-17 in a closely-fought semifinal that lasted one hour and 14 minutes at Odense Sports Park in the USD 650,000 Super Series Premier event here.
This was 20-year-old Hyderabadi's first win against Marin in their last four outings.
Sindhu, a twice bronze medallist in World Championships, will now take on former world number one and reigning Olympic champion Li Xuerui -- ranked fourth in the tournament -- of China in the summit clash tomorrow.
Sindhu had never gone past the semifinals in the Super Series or Super Series Premier events.
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Both Sindhu and Marin began the match on an equal footing before the Indian grabbed five points on the trot to cruise to a 8-3 lead before Marin closed the gap gradually.
Sindhu, the lone surviving Indian, kept the lead as the two rivals exchanged their services for a while before the Indian took four straight points to reach to game point before pocketing the first set 21-15.
But suddenly, Sindhu lost her concentration as world number two Marin made it 6-6 before zooming to 11-6 by taking five points on the trot.