Chelsea restored their three-point lead in the Premier League - but only after an unconvincing win over Portsmouth.
Nicolas Anelka side-footed Chelsea ahead after good work from Alex as the home side dominated the first half.
But Pompey, managed by former Blues boss Avram Grant, levelled through Frederic Piquionne's fierce finish and even had chances to go ahead.
Frank Lampard saved the hosts' blushes with a 79th-minute penalty after Marc Wilson's lunge on Branislav Ivanovic.
Teenage substitute Fabio Borini could have made the win more comfortable had he not dragged his shot narrowly wide - but that would have masked a curiously flat performance by the league leaders.
Ancelotti impressed with Chelsea strength
They can, at least, celebrate their first win in five matches as they look ahead to Sunday's trip to West Ham, but Carlo Ancelotti must be concerned by his side's stuttering form.
Grant was given a warm reception on his return to Stamford Bridge, but the home fans were probably not feeling quite so charitable when the Israeli's side threatened to deal a stunning blow to the Blues' title chances.
After Mick McCarthy controversially fielded a weakened Wolves side at Manchester United on Tuesday, eyebrows were raised when Grant made seven changes to the side that snatched a late draw against Sunderland at the weekend.
But in reality, two were enforced, David James missing out through illness and Younes Kaboul serving a suspension, while Papa Bouba Diop, Kevin-Prince Boateng and Steve Finnan all returned to the side after injury and illness.
Having watched his side concede three goals at home to Everton on Saturday, Ancelotti brought Alex into his defence to replace Ricardo Carvalho.
Up front, Didier Drogba was also absent, nursing a back injury, but he was not missed in a one-sided first half, while under-pressure keeper Petr Cech was barely tested.
Chelsea dominated the opening 20 minutes, with Michael Ballack and Anelka going close and Lampard spooning a good chance into the crowd.
Anelka finally made the breakthrough, Alex galloping out of defence and holding off Wilson's challenge on the right before cutting back for the Frenchman to score on his 100th appearance for the Blues.
Grant sees positives in defeat
In driving sleet and freezing conditions, it was hard not to feel sympathy for the under-employed Cech, and the keeper almost fluffed his opening lines on 35 minutes when he spilled a well-struck Piquionne shot - but Hermann Hreidarsson could not control the rebound.
Portsmouth, though, improved after the break, as Chelsea appeared to slip into complacency.
The Blues' set-piece defending has come under intense scrutiny in recent weeks, so it was no surprise that the equaliser came from a free-kick, but there was a slice of good fortune for Portsmouth.
Jamie O'Hara's 30-yard effort deflected off the end of the wall, and then off Ashley Cole, before falling into the path of Piquionne, who smashed a glorious finish past Cech.
Grant helped me through 'difficult time' - Lampard
The home fans, quiet until that point, suddenly ramped up the noise, but Chelsea continued to toil with Drogba's absence suddenly keenly felt.
Boateng twice had chances to give Pompey a shock lead, but was denied first by a desperate Ashley Cole block and then lost his footing with the goal gaping.
The introduction of Joe Cole for the injured Deco injected some welcome pace and creativity into the Chelsea side.
And the goal they craved arrived with 11 minutes remaining, Wilson diving in wildly on Ivanovic as the right-back burst into the box.
Lampard, whose previous penalty effort was saved in the defeat by Manchester City, made no mistake this time, hitting it high into the roof of the net.
Chelsea boss Carlo Ancelotti: "We did well in the first half, played good football, not with great intensity, but good.
"But after they equalised, we lost our confidence, our composure, and it was a difficult game in the second half.
"It was important to win, to maintain our lead. It was not our best moment because we are lacking. But I think we deserved to win."
Portsmouth boss Avram Grant: "If we continue to play like this, we will be off the bottom.
"We played tactically well. I'm very very proud of our performance today.
"It was nice [to return to Chelsea], the supporters have been very good to me since I left, and I will always have a bond with this club."
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