The goal that made the difference was a worthy winner, Sebastian Kneissl ending a five-man move midway through the first-half with an instinctive finish for his second goal since returning from loan.
The rest of the game was comfortable for the home side, a period towards the end of the second-half apart. In truth, Mick McGiven's side never looked like coming away with anything less than three points.
It was an evenly-matched contest in terms of experience, Portsmouth with just one player blooded in their first-team. With Robert Huth having moved back into senior action after his reserve outing last week, Adrian Pettigrew returned to the Chelsea defence and there was a first reserve game of 2005 for Ben Hudell, in at left-back for the injured Sam Tillen.
Hudell would make an important contribution towards the end of the game but it was his mistake after almost ten minutes of constant Chelsea possession that led to the first chance of the game.
He allowed the ball to bounce across the area, Portsmouth's Liam Horsted seizing the opportunity to shoot on target but Lenny Pidgeley stood tall and blocked.
A minute later came the first Chelsea effort of note when Kneissl, receiving Anthony Grant's pass, turned his marker well and unleashed a powerful strike which Guatelli dived to save.
There was no touch required from the Portsmouth keeper when Danny Hollands tried his luck a few minutes later but his well-executed volley was a good effort that flew a yard or two wide.
Next came the goal, the attack beginning with Grant's foraging before the ball passed through Steven Watt and Craig Rocastle to Nuno Morais, who was looking increasingly assured in the right-back role.
Cutting in from the flank, he drilled a low cross to where Kneissl had taken up a position outside the near-post, deflecting the ball on target and in. It was a fine example of the goal poacher's craft.
There were two enforced Chelsea changes at half-time. One was in goal where Yves Ma Kalambay replaced Pidgeley who appeared to have a problem with a thigh muscle approaching the break. The other saw Phil Younghusband come on for an ill Filipe Oliveira.
A mistake in the Portsmouth defence allowed Kneissl to run onto Woodards pass five minutes into the half but his attempted chip had height to spare.
It then took a smart save from Guatelli to keep out the young German after a sharp swivel and shot having received Woodards pass with back-to-goal. Younghusband was only successful in turning the rebound onto the roof of the net.
The Portsmouth defence were finding it tough at times to cope with Kneissl and were no doubt relieved to see him replaced by Joe Tillen on 68 minutes.
The Chelsea performance became more laboured as the half progressed but a Ma Kalambay dash to hack clear a dangerous break was the only moment of panic until the 72nd minute when the strong-running Horsfeld raced away down the left.
He crossed low past several flailing legs before substitute Tom Settle connected. Hudell was in the right place at the right time to block on the line and preserve a deserved victory.
Chelsea Reserves:
Lenny Pidgeley (Yves Ma-Kalambay); Nuno Morais, Adrian Pettigrew, Steven Watt, Ben Hudell; Craig Rocastle, Anthony Grant, Danny Hollands (Capt); Filipe Oliveira (Philip Younghusband), Sebastian Kneissl (Joe Tillen), Danny Woodards.
Portsmouth Reserves:
Guatelli, Vongas (Hartman), Cooper, Harris, Pearce, Angus, Cousins, Wilson, Keene, Eyles (Settle), Horsted.