Carlo Ancelotti admitted Napoli should have been more clinical in their goalless draw with Genk, but defended his decision to leave out some key attacking players.

Dries Mertens was dropped to the bench and Lorenzo Insigne was not part of the squad as Napoli played out a stalemate in Wednesday's Champions League clash at the Luminus Arena.

The Serie A side hit the woodwork three times in the first half and had a shot saved off the line, with Arkadiusz Milik particularly guilty of wasting some promising opportunities.

Napoli beat Liverpool in their opening Group E fixture and remain top of the standings, though Ancelotti acknowledged his side could have been further in front of the chasing pack.

"We knew this game could be tough," he told reporters. "We should have simply been more clinical, that's all I have to say about the team's performance.

"The goal remains to finish first, even if in reality first or second it doesn't change much.

"Every game in the Champions League is difficult, but we're in a good position after two games."

Asked about his decision to overlook Insigne for selection, with Hirving Lozano partnering Milik up top, Ancelotti said: "He was lacklustre in training so I preferred to keep him out and prepare him for the next game.

"Milik had his chances. He and Lozano had a good game.

"Unlike Italy, the games are much more intense in Europe. We have to improve in the small details, at the moments that can cost us."

Genk were beaten 6-2 by Salzburg two weeks ago and captain Mbwana Samatta was pleased with the response from his side.

The Belgian hosts tested Alex Meret twice in the first half but never really looked like coming away with all three points.

"We played against a tough opponent but it was important that we showed character today after the bad result in Salzburg," Samatta said. 

"It is important for us to get a draw in the Champions League and start from that."