Academy: Watford 3-1 Crawley Town
Goals from Josh Doherty, Alex Jakubiak and Kurtis Cumberbatch saw the Hornets move level with Pompey on four points from their opening two matches.
The visitors scored after five minutes, but saw the goal chalked off for offside.
Wing-back Harry Kyprianou crashed an effort off the Red Devils' crossbar before American goalkeeper Arrie Ammann denied Town's Scott Melville impressively.
Watford's opener came direct from a free-kick when Doherty curled over the wall after midfielder George Byers was fouled.
The Hornets' second game as the result of direct football, Ammann sending a long clearance towards Cumberbatch who headed down for Dennon Lewis who played in Jakubiak to finish.
Watford killed off the contest when skipper Ryan Hope clipped a ball down the line for Jakubiak to chase, the striker crossing for Cumberbatch to side-foot home.
Having been denied earlier by Ammann, Melville got Crawley's consolation goal late on, finishing Tope Fawody's cross.
Watford: Ammann; Hall, Young, Kyprianou; Barnum-Bobb (Bawling 45), Byers (Westlake 83), Hope (c), Cumberbatch, Doherty; Jakubiak & Lewis (Otudeko 83).
Subs not used: Martin & Dillon.
Group 5 Table:
1. Portsmouth P2, GD+2, PTS 4
2. Watford P2, GD+2, PTS 4
3. Barnet P2, GD+1 PTS 3
4. Crawley Town P1, GD-2, PTS 0
5. Wimbledon P1, GD-3, PTS 0