BERGAMO, Italy — Enzo Maresca admitted he was "worried" by Chelsea's slump, after Atalanta's 2-1 victory in the Champions League extended the Blues' winless run to four matches on Tuesday.

Joao Pedro's first-ever Champions League goal put Chelsea ahead in the first half in Bergamo.

But, the Blues lost their momentum after the interval, and former West Ham striker Gianluca Scamacca grabbed the equalizer, before Charles De Ketelaere, who provided the assist for Atalanta's leveler, capped his influential display with a late winner.

Chelsea found it impossible to subdue Belgium forward De Ketelaere, who has six goals and seven assists in his last 12 Champions League appearances.

Since its memorable 3-0 win over Barcelona in the Champions League on Nov 25, Chelsea has gone four matches without a victory in all competitions.

Maresca's 11th-placed side has 10 points from six league phase games, and its bid for a top-eight finish — which guarantees automatic qualification for the last 16 — is now on a knife edge.

Chelsea's last two matches are in January at home against Pafos and away to Napoli.

"I am always worried. Two points in four games is not what we expect, and we have to do better," Maresca said.

"First half we were winning, we were good and controlling the game. After we conceded the first goal, we lost control of the game a little bit.

"We had different chances to make it 2-0, and when we conceded 1-1 the dynamic changed a little bit.

"We are playing every two days. For a moment we looked like we struggled a bit. Now it's about trying to win the next game."

Third-placed Atalanta has enjoyed wins over Club Brugge, Marseille, Eintracht Frankfurt and Chelsea this season, as it chases a last-16 berth.

Maresca made five changes from Saturday's 0-0 draw at Bournemouth in the Premier League, with Cole Palmer rested after his return last weekend from a two-month injury absence.

There was no place for Chelsea's emerging teenage star Estevao, who has three Champions League goals this season and starred against Barcelona.

Maresca's reshuffled lineup nearly fell behind inside five minutes, when Ademola Lookman weaved into the area for a cross that deflected off Chelsea goalkeeper Robert Sanchez before it was hacked to safety.

Lookman was a thorn in Chelsea's side, and it took a last-ditch block from Josh Acheampong to deny the former Everton winger, with the rebound curled just over by Marten de Roon.

Chelsea made the most of that escape, taking the lead in the 25th minute.

When Pedro deftly slotted in Reece James' cross from four yards, the goal was initially disallowed for offside, before a VAR check ruled in favor of the Chelsea forward.

It was only Pedro's third goal for Chelsea in his last 16 games in all competitions.

James should have doubled Chelsea's lead when he smashed just wide from the edge of the area early in the second half.

It was a crucial miss, and Atalanta made James pay as it drew level in the 55th minute.

De Ketelaere's cross to the far post caught Chelsea's defenders flat-footed and Scamacca made a perfectly timed run to head home from close range.

Scamacca almost punished Chelsea's sloppy marking again moments later with a glancing header that Sanchez saved at full stretch.

Maresca responded to Chelsea's lethargy by sending on Alejandro Garnacho, who forced an immediate save from Marco Carnesecchi.

But Chelsea was creaking at the back, and De Ketelaere snatched the winner in the 83rd minute, advancing unchecked to the edge of the area for a powerful strike that deflected in off Marc Cucurella.

AFP