A solitary Omer Toprak goal was enough for Leverkusen to beat Real Sociedad 1-0 on Tuesday night to go through, and with the side performing well in the Bundesliga Kiessling believes that there is a lot to come from them.
"We produced a performance from which we could, probably should, have scored a few more goals," he told UEFA.com. "Most of our chances came from open play but the goal was from a corner and we hit the bar with a free kick so there was a mix of opportunities and there was quality involved in them all.
"The fact we didn't convert more chances is something that will stick in our throat a bit; it's something to learn from.
"We have registered good results in the league, cup and now the Champions League away from home after that defeat by Manchester United, and that's an indication of us growing as a team. We are only just becoming aware of what we can achieve and what our potential is."
Leverkusen are four points behind Bayern Munich in second place in the Bundesliga.