The team from DR Congo, who scored in the 21st minute through midfielder Mbenza Bedi, will have an even steeper climb next Tuesday when they meet Internacional of Brazil.
"Each time you pass a peak, the mountain becomes higher," Mazembe's Senegalese coach Lamine N'Diaye told the post-match news conference.
"So, to use a cycling term, this was a first category col. Now we're going to attack a major climb," he said.
"They are a dimension above," he added of Internacional, winners of South America's Libertadores Cup and favourites to reach a final against Inter Milan.
Mazembe played with 10 men for the last nine minutes after their Zambian midfielder Stoppila Sunzu was sent off for a second yellow card offence.
Pachuca's Argentine coach Pablo Marini said his team had dominated for at least an hour but failed to put away their many chances.
Asked if the result was a 'disaster' for Pachuca, Marini said: "Obviously, yes. We knew about Mazembe's strengths but we didn't expect to lose.
"I think the way the match went, we shouldn't have lost. But, unfortunately, we made a mistake with the goal and did not have the calm to put away our chances."
KEEPER'S PRAYER
Mazembe goalkeeper Muteba Kidiaba, who knelt and prayed on his line before the start of each half, led a charmed life as Pachuca threw everything into attack.
The African side had three more players, including Bedi, booked as they clung desperately to their lead and, but for some wayward finishing, they might have made the win secure in several second-half counter-attacks.
N'Diaye had no complaints with Japanese referee Yuichi Nishimura, saying: "The cards were well deserved, (the players) were late (in their challenge) to the ball."
The match ultimately hung on a spell of less than two minutes midway through the first half.
Concacaf champions Pachuca almost took the lead in the 19th minute when Argentine midfielder Alejandro Manso's shot from outside the box on the left hit the inside of the far post and crossed the front of goal before spinning out.
In the Congolese side's next attack, Bedi ran into the box on the right onto a fine through ball from forward Mulota Kabangu to beat Colombian goalkeeper Miguel Calero at the near post for the winner.
It was Mazembe's first victory in two appearances in the competition having lost twice in Abu Dhabi a year ago.
Pachuca, in their third appearance in the competition but with only one win in five matches, will play for fifth place against the losers of Saturday's quarter-final.
Seongnam will be looking to at least emulate fellow South Korean side Pohang Steelers who took third place in Abu Dhabi last year.
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