Jonas went to work early on, landing with some nice counter-punches. However, the judges rewarded Underwood for her work rate by scoring the first round 4-3 in her favour.
There was nothing between the pair as they stood toe to toe in the centre of the ring. Jonas was finding the target with some dangerous hooks, which was enough to overturn the deficit and lead by one point after two rounds.
The American surged forward at the start of the third, but was matched by Jonas, who displayed some sharp reflexes to pick her off. The Brit had now found her rhythm and was scoring well.
Jonas had opened up a four-point cushion as they entered the final round, but Underwood piled on the pressure from the bell as she fought to keep her Olympic dream alive.
The American landed with two straight left hands that stung Jonas, whose hands were low and inviting Underwood inside.
Jonas then forced a standing count on Underwood, which gave her some time to catch her breath in a frenetic final round.
The judges rewarded Jonas with a 21-13 decision.