The F1 chief executive and other officials have sworn for months that, contrary to eyewitness reports and speculation, the new Buddh circuit would be ready for its late October race date.
But Ecclestone told the Guardian on Saturday: "I was pessimistic a fortnight ago. I was sent pictures of the track and I thought Christ, we're not going to get it finished in time.
"But what they have done in the meantime – and I've been sent more recent pictures – is incredible."
Added HRT's Indian driver Narain Karthikeyan, who has been in Delhi this week: "There is still some work being done on the landscaping but everyone is working 24-7 to make sure everything will be fine on the day."