Visiting CS Sfaxien held Vipers to a goalless draw in the first leg of the 2nd 1/16th round at St. Mary's Stadium, Kitende on Saturday.
The Ugandan champions put in a good show on the day but were largely denied by poor finishing and good goalkeeping by Ayman Dahmen.
In the first half, Kenyan import Noah Wafula should have put the Venoms in the lead but missed connection from Yayo Lutimba's low cross that beat the CS Sfaxien defence.
Moments later, he miscued another effort when faced with goalie Ayman, while another opportunity went begging as Tito Okello headed at the crossbar.
In the 23rd minute, Vipers goalie Fabien Mutombora also tipped Achraf Zouaghi long range after a long range shot into the corner.
Vipers captain Taddeo Lwanga foiled an attack by an opponent, raced past a couple of opponents and set Moses Waiswa only for his effort to be blocked for a corner which CS Sfaxien defended well to end the first stanza goalless.
After recess, Vipers technical bench led by Michael Ouma made a forced change with Brian Nkuubi taking the place of injured Bashir Asiku with Geoffrey Wasswa taking a place at the centre back to partner Livingstone Mulondo.
Abraham Ndugwa also came on for Wasteful Noah Wanyonyi Wafula who had an off day as the Vipers SC went all out to attack the Tunisian's.
With all the dice for Vipers on the pitch it looked as if it was about time for the home side to lead but Tito Okello had two headers saved off the line by goalie Ayman while Taddeo Lwanga ball was also cleared off the line as the hosts pressed for the opener.
The visitors also had chances on the counter but at the final whistle, the game ended goalless.
CS Sfaxien coach Ruud Krol was delighted with leaving St. Mary's stadium, Kitende without defeat although he would have wanted his side to score.
"It was a tough match for us in different conditions and on artificial turf. Back home, its winter now and it was always going to be difficult and we are also used to playing on natural grass.
"They also had chances and we defended well but sometimes we were naïve. Thanks to our goalkeeper because I think he had a very a good game today," Krol said.
On his part, Vipers coach Ouma said: "It was a good match; unfortunately, we failed to convert the many chances we created. We shall go to Tunisia with a positive attitude because we have nothing to lose. We can score in Tunisia and things will change".