After Napoli extended their advantage at the summit with a 2-1 victory over Juventus on Saturday, the pressure was right back on second-placed Inter but they responded in style.
Napoli are now top with 53 points, three ahead of Inter while Atalanta are third with 46.
"There is no fear, just a lot of respect for two very good teams, who have fine coaches and play good football. I said in July that this would be an open title race and this is what is happening," Inzaghi told Sky Sport Italia.
"Napoli and Atalanta are doing something extraordinary, as are Inter."
Last season's champions Inter had few problems sweeping aside Lecce, with the result sealed early in the second half and Inzaghi substituting a number of key players just after the hour mark.
"We had a great approach to the game and I congratulated the team for that, but it was not easy, as we returned home on Wednesday at dawn," Inzaghi said referring to their midweek win at Sparta Prague in the Champions League.
"These lads have been playing every three days for months, it will continue like this, so we need to keep it going."
Inzaghi had plenty to celebrate after the victory, their eighth straight away league win, with the Inter coach setting a Serie A record for the fewest matches to reach 200 wins.
"There is a lot of daily work in training behind these records and I am very fortunate to have worked for clubs like Lazio and Inter who gave me carte blanche, with players who were prepared to follow me," he said.