Thiago Motta and Angel Di Maria were sent off late in the 3-1 defeat on Sunday as PSG were left second in the table.
Monaco are three points clear at the top and have a game in hand, leaving PSG's hopes of a fifth consecutive league title hanging in the balance.
Emery rued the defeat but said his team would continue fighting as long as finishing top was mathematically possible.
"We lost a good opportunity," he told a news conference.
"It is a very hard result for us. We lost three very important points, but as long as we have a chance to play for the title, we will continue to push."
Mario Balotelli and Ricardo Pereira struck for Nice before Marquinhos gave PSG hope just after the hour-mark.
Motta and Di Maria were shown straight red cards either side of Anastasios Donis' 92nd-minute sealer as the visitors imploded.
Emery said: "We are three points ahead of Nice.
"We will continue to fight for first place and try to secure second."