The Democratic Republic of Congo team will tackle Internacional of Brazil for a place in the final thanks to Mbeza Bedi's 21st-minute goal.
Bedi struck just two minutes after Pachuca's Alejandro Manso had hit the post with a left-foot volley.
He latched on to Mulota Kabangu's pass before beating Miguel Calero in the Pachuca goal.
The Mexicans hit the woodwork again on the hour mark when Raul Martinez unleashed a fierce drive but Mazembe hung on despite Sunzu's second yellow card nine minutes from time.
"The match was very difficult for us, as we expected it would be," said Mazembe coach Lamine N'Diaye.
"Our aim now is to develop and to play better in the coming games. We won tonight's match due to the bravery and courage of our team, but technically we could have been better.
"Internacional are a strong team and we are looking forward to playing them. We will work hard and give our best to try to win."
The tournament's other semi-final will feature European champions Inter Milan against either Al Wahda of the UAE or Seongnam Ilhwa Chunma of South Korea who meet on Saturday.