Brayan Leon scored the only goal of the game with powerful header from close-range in the opening minutes of the second half at the Hammadi Agrebi Stadium on Sunday night.
The two sides clash in the second leg at Loftus Versfeld on Saturday and Beaumelle knows his side have a fight on their hands to defy the odds and reach the Champions League final.
"I am very disappointed, not just by the defeat, but especially because the scoreline doesn't reflect the match," Beaumelle said on Pan-Africa Football.
"We were more aggressive, more committed, and we managed to create several clear chances," he added.
Beaumelle, however, added that the tie is far from over. "Nothing is decided at this stage, no team has qualified or been eliminated," he said.
"The decision will be made in the second leg, where we absolutely must be more efficient and more clinical."
Mamelodi Sundowns coach Miguel Cardoso says his side need to take control of their CAF Champions League semi-final after holding a narrow advantage from their first leg.
Junior Khanye has thrown his weight behind Mamelodi Sundowns defenders Keanu Cupido and Grant Kekana, insisting their recent performances have made a strong case for inclusion in the South Africa national football team.
Mamelodi Sundowns coach Miguel Cardoso has warned that his side's hard-fought 1-0 victory over Esperance de Tunis away from home will count for little if they fail to finish the job at home.
Mamelodi Sundowns recorded a massive 1-0 win over Esperance Sportive de Tunis in the first leg of their CAF Champions League semi-final in Tunisia on Sunday night.
