Kevin Gameiro's 77th-minute strike gave the La Liga side a 2-1 victory in Portugal on Thursday and Emery was delighted with the spirit shown by his players.
When Bruno Miguel's strike just after the hour levelled the scores, Estoril looked favourites to go on and win, but Gameiro's late strike - his third for the club - gave Sevilla three valuable points in Group H.
Emery believes his team can do well in the competition this year and stressed the importance of their early win.
"This is an important satisfaction because we have begun with a win and got going," he said. "We have settled on having optimistic ambitions in this competition.
"During the first half we had control and had done enough to move ahead, but we lacked the goals.
"The second half was in the same vein and we struggled to open the scoring, but when we thought that the game was in our favour, it was not.
"They equalised and left us hit, but the reaction was good. The team reacted and made it 2-1 for the game. We had many attacks to make the game easier, but we were tired by the end.
"The bottom line is that we are deserved winners and we had the chance of winning better."
Sevilla - who won the Europa League in 2006 and 2007 - face Freiburg in their next group game next month.
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