Academy: Portsmouth 2-2 Watford
The Hornets were pegged back by an 88th minute equaliser from hosts Portsmouth, Connor Chaplin the scorer for Pompey.
Lewis Horburgh put the hosts ahead just before the break, but Josh Doherty levelled for Dave Hughes' men to send the sides into half-time at 1-1.
Hornets' striker Alex Jakubiak put the Golden Boys in front in the second-half before Chaplin's late goal denied the young yellows an opening win.
Watford: Wilks; Doherty, Young, Hall; Barnum-Bobb, Hope, Rosenthal (Cumberbatch, 58), Cox (Westlake, 84), Kyprianou; Lewis (Bawling, 66) & Jakubiak.
The Youth Alliance Cup is a group format, with Watford in Group Five:
Barnet P1, W1, GD +3 PTS 3
Portsmouth P 1, D1, GD 0 PTS 1
Watford P1, D1, GD 0 PTS 3
Crawley P0, PTS 0
Wimbledon P1, L1, GD -3 PTS 0
Watford's next Youth Alliance Cup fixture is against Crawley Town, whilst their next league game is at home to Stevenage on Saturday morning.