Al Sadd were beaten 1-0 by Sepahan in the first leg of their last-eight meeting, but the Iranians were penalised for fielding an ineligible player.
The Qataris were subsequently awarded the game 3-0 by the Asian Football Confederation and despite a 2-1 home defeat in the return, progressed to face the Koreans.
"We are not a weak team," said Fossati ahead of Wednesday night's clash at Suwon World Cup Stadium.
"We just followed the rules and the opposition made a mistake. We have been waiting for this game to face a strong team like Suwon and we are excited. I believe that as we have confidence in our own ability, we can have a good game.
"The possibility of winning tomorrow's [Wednesday] match is 50-50. The game starts at 0-0 so nobody can know who will end up victorious."
Al Sadd defender Abdulla Koni is suspended while Suwon will be without right-back Oh Beom-seok.
Bluewings boss Yoon Sung-hyo is keen to make home advantage count.
"To get good results in the AFC Champions League you should aim to win at home to make the away matches a little more straightforward," he said.
"Of course we will be looking to attack and score as many goals as we can, but away goals could prove to be crucial so we will do our best not to concede any goals.
"We are looking forward to the game and we know that Al Sadd are a big club."
The other semi-final sees Jeonbuk Motors travel to Al Ittihad with boss Choi Kang-hee eager for his side to continue their free-scoring ways - they have found the net 26 times in nine games - at Prince Abdullah Al Faisal Stadium.
"I am fully aware that we are facing a strong team playing on home turf and I watched the two matches of the quarter-final with Seoul and recorded some of my observations," said Choi.
"There are strengths and weaknesses in the Al Ittihad team, I know them well, I will seek to exploit weaknesses to surprise the opponent and I am optimistic about the outcome of a positive result.
"The goal will not be to defend at all and I will look for a win that will facilitate our mission in the second leg at home. The Al Ittihad team are well-known and have international players but we will be playing to win."
Al Ittihad manager Dimitri Davidovic was equally upbeat over the Saudis chances.
"We have an increased rate of fitness and harmony among the players after playing in several stages in the league unlike the two matches against Seoul (in the quarter-finals), which we played after one game, so I'm confident of winning," said the Belgian.
"The game will be on our pitch, our fans will fill the stadium and galvanise the players with their support and motivate them to play their best.
"I have great confidence in my players and I hope we play at a high level and get the right outcome. Most important is not to have conceded a goal, because that would be difficult for our away trip, and to score as many goals as possible."