Danny Rose flew in to prevent Jordan Ayew’s shot from troubling Foster just before half-time, prompting loud cheers from the Rookery Stand behind him.
The visitors enjoyed most of the possession in the first 45 but weren’t able to make it count, as the teams went in goalless at the break.
It was Watford who had the first major chance of the second-half, as Troost-Ekong’s downward header called Jack Butland into action for the first time. The hosts’ attacking threat continued with substitute Emmanuel Dennis in the thick of the action. The Nigerian cleverly flicked the ball into Sema, but Fletcher’s eventual shot went wide.
As the game wore on and shots on goal became limited, a penalty shoot-out became more and more inevitable, but Fletcher had other ideas.
With four minutes left on the clock, Hernandez received the ball in familiar territory on the left side, just outside the 18-yard box. Rather than going for similar glory to the Premier League opening day, the Colombian opted to cross, and the debutant slid in to poke home from close range and break the deadlock.
Christian Benteke netted late on in the friendly between the two sides earlier this month, but this time Ben Foster caught his effort and prevented a dramatic late equaliser.