But Jose Mourinho will be heartened by the fact his side still have a game in hand on United, who are also due to visit Stamford Bridge in April.But it was not all plain sailing for the Blues on the south coast as they quickly found themselves on the back foot in the early stages.
Cech had to be at his best to keep out a smart right-footed shot from Lomana LuaLua after eight minutes.
Chelsea looked distinctly out of sorts in the difficult conditions despite having the majority of the possession.
But they showed they were dangerous on the break when Drogba and Andriy Shevchenko combined down the left flank.
The opportunity looked to have gone when referee Mark Halsey got in the way, but the loose ball rebounded to Ashley Cole, who lofted it just over the crossbar.
Shevchenko disappointed when he sent a shot into the side-netting after being put through by Arjen Robben.
However, the Ukrainian captain made amends moments later by supplying a sublime ball for Drogba. But the Ivorian's accuracy was off key for once and he fired high and wide to earn the derision of the home fans.
In the 27th minute Kanu tried his luck from 25 yards, but Cech was equal to his low drive and collected the ball comfortably.
Five minutes later Chelsea took the lead and it was the in-form Drogba who fired home his 29th goal of the season.
Robben laid the ball back to Cole and the England defender's long cross eluded the Pompey defence, but not the lurking striker, who despatched a right-footed volley across James and into the bottom corner.
It was rough justice on the home side, who had more than matched the champions until Drogba's intervention.
Nine minutes before the break Taylor got clear down the right flank for Portsmouth, but his low cross was well taken by the diving Cech.
Portsmouth continued to take the game to Chelsea but poor finishing prevented them from scoring the equaliser they deserved on their first-half display.
Two minutes before the break Richard Hughes had the chance to test Cech when he collected a loose ball 20 yards from Chelsea's goal.
But the Portsmouth midfielder could not keep his head over the ball and it flew over the crossbar and into the massed ranks of the Chelsea fans behind Cech's goal.
Chelsea squandered a great chance to put themselves two in front in the 46th minute when a lovely move ended with Drogba putting Robben through on goal.
But the Dutch winger's shot hit the upright when a cross would have found the waiting Shevchenko completely unmarked in the six-yard box.
Chelsea were now completely in control and Lampard demonstrated their growing confidence by unleashing a 25-yard drive that James did well to hold as it threatened to reach the top corner of the net.
In the 57th minute Robben was jeered by the crowd when he went down under a challenge and then appeared to be hit by a missile thrown from the crowd.
The Dutch winger was then booed every time he collected the ball as Chelsea looked for the decisive goal.
Portsmouth appeared to have run out of ideas in attack as Chelsea, inspired by the giant frame of Michael Essien, coped easily with everything the south coast side could muster.
But it was to take two sensational saves from Cech to protect their lead.
In the 77th minute he saved brilliantly at point-blank to stop a header from Andrew Cole and he repeated the feat seconds later to turn another headed effort from Kanu around the post.
Pompey paid a heavy price for their failure to beat Cech when Lampard's long pass was headed into the path of Kalou by Cole and the Chelsea substitute slipped the ball beyond James to keep their title dreams alive.
Mourinho happy with 'big victory'
Mourinho was relieved Chelsea's bad luck with injuries this season had not
surfaced again against Pompey.
He said: 'It was a big victory today and no injuries. That is almost unique
this season.'
Petr Cech's absence through injury earlier in the campaign was perhaps the
most significant one.
Mourinho said: 'If the best keeper in the Premiership had not been out for
four months how many more points would Chelsea have? He is a keeper who gives us
10 to 15 points a season.'
Asked if he thought United were currently riding their luck after back-to-back
last-minute wins against Fulham and Liverpool, Mourinho said: 'Are they lucky?
Yes. They win the last minute and they have no injuries so the Gods are with
them.
'But we have to be honest and fair with them and give them what they deserve
which is big respect to the manager and the players.
'Our job is to keep winning matches and get back to six points (behind) and
have the pressure on them. We have to win all our matches and wait for them to
drop some points.'
Frank Lampard echoed Mourinho's defiance on the title race.
He said: 'It is still doable and we have to keep going. We saw the game (at
Anfield) earlier and we could have come out with our heads down and not got a
result. But we didn't. We battled well, got two goals and the win.
'Man Utd are doing great. They have had two great late wins but it doesn't
always go your way. We have to keep plugging away and try to close the gap.'
Lampard agreed with Mourinho's assessment of Cech's importance to the Chelsea
cause, adding: 'He's world class. If Petr hadn't been injured so long we'd be
up there with Man Utd.'
Cech, who has kept seven clean sheets in 10 matches since returning from a
head injury sustained at Reading, insisted: 'It (the title race) is not
finished.'
Portsmouth manager Harry Redknapp was pleased with the effort of his players and
praised Chelsea goalkeeper Cech for some vital saves.
'I thought we played well and was a bit disappointed to be 1-0 down at
half-time,' Redknapp said.
'In the second half we were a lot stronger and I thought there was an
equaliser there for us but the keeper has pulled off a couple of fantastic
saves. He is a world-class goalkeeper.
'We got caught pushing forward for their second but overall I was very happy
with the players.
'Chelsea are not champions for nothing, they are a great, great side. Them
and Man United have been fantastic all season and to match them tonight I was
delighted with the players.'
Portsmouth 0 - 2 Chelsea
