Watson (11-2) was unfancied by those sitting ringside, having lost his last fights and not won since 2014, but he knocked down Nassa (10-1) three times before the referee waved off the contest.
From the outset, it was Nassa who set the early pace of the contest with his neat jab and accurate counters seeing him take the opening two rounds, much to the delight of the passionate and loud crowd.
Watson enjoyed some rare success in the third who showed his power with some heavy shots late on, although a nasty cut to his nose, which was ruled an accidental clash of heads, left him hurt.
Rounds four and five saw Nassa reassert his early dominance and, when fighting on the outside, he looked in control. Watson impressed in brief moments, troubling the home fighter when in close proximity, but it was never enough to win the rounds.
At the halfway stage, Nassa was up on the cards and looked set to jab his way to another win. But, then came round six.
The Sheffield fighter started well and landed a few significant blows upstairs but, with 30 seconds remaining, a flash right hook counter from Watson put Nassa down (for the first time in his career), stunning the crowd and, most significantly, Nassa.
Watson launched on attack on the 22-year-old on the ropes in the dying seconds but couldn’t find the early finish just yet.
Clearly sensing that the end for Nassa was imminent, Watson disregarded his defence to stand toe-to-toe and trade with Nassa in a barn-stormer of a round in the seventh, in what was easily the best three minutes of action seen all evening.
Both fighters took extraordinary punishment in a battle of the chins and somehow both stayed on their feet with Nassa rallying to finish with a nice combination on the bell.
Watson found particular success with the right upper-cut early on in the eighth and clearly held the advantage in terms of power. Once more though, Nassa showed grit and rocked his Guernsey opponent with a counter left hand late on.
The ninth came around, for the first time in either fighters’ careers, and by now it was a battle of the wills. Further brawling ensued in the opening stages and by now Watson’s power seemed to be showing in a round that was clearly his.
It was Watson who had won the battle of the resilience and stamina though as in the tenth and last he twice sent, firstly with another big right hook before forcing Nassa to ground with another attack on the ropes.
Nassa rose to his feet twice but the referee saw enough the second time round, waving off the contest and cueing some huge celebrations from Watson and his team.
Jacob Kilbride is a staff writer for World Boxing News. Follow on Twitter @jacob_kilbride