India must tighten up their fielding and be "ruthless" when they face Sri Lanka in the Champions Trophy at the Oval on Thursday, skipper Virat Kohli said.

The defending champions opened their Group B account with an emphatic 124-run victory over arch-rivals Pakistan on Sunday, but drew criticism for their lacklustre fielding performance.

"Sometimes, starting with the tournament, creating momentum as a team, you can misfield, you can get overexcited," Kohli told a news conference on Wednesday.

"There can be lapses as well, especially when the team is seven or eight down and you know that the game is going to end soon. We have addressed the guys and have asked them to be ruthless and finish games off."

Kohli also told his team mates not to get carried away by the praise that has come their way since they beat Pakistan. Despite the occasional fielding error, India dominated the rain-shortened match.

"I don't think when you are playing good cricket, you should look too much into the praise factor," he added.

"As a team, we really don't focus on these things and we try to maintain the same kind of intensity and momentum regardless of how we are playing.

"At this moment we are playing some good cricket but that doesn't mean that we play with arrogance... In this sport, anyone can upset anyone on their given day... you certainly cannot even think of taking anyone lightly."

Both India and South Africa have two points apiece having won their opening encounters, but India top the group thanks to their superior run rate.