Mark McCall admitted Saracens had to dig deep in their 29-20 Champions Cup quarter-final victory over Northampton at Allianz Park.
England wing Chris Ashton crossed in the 69th minute to finally put the tournament favourites in control before Chris Wyles produced a late try to sweep the game beyond Saints' reach.
"This was a complex match for us. In the first half we didn't play anywhere near to the level that we can," director of rugby McCall said.
"We just needed to find a way to turn the energy of the game around. Give Northampton a lot of credit because they put us under all sorts of pressure at the breakdown.
"Good teams find a way of getting the job done. We were nowhere near our best and the last few weeks in the Premiership were almost too easy for us in a way.
"These games are different, there's a lot at stake. This was the first game of the season where it's on the line and if you lose you're out of the competition.
"Maybe we played not to lose a bit and we were lucky to be only four points down at half-time.
"We know we'll have to improve a lot of things against Wasps in the semi-finals."
Ashton, who scored his third try in two matches since returning from a 10-week suspension for making contact with the eye area of Ulster's Luke Marshall, admitted Saracens had been pushed hard.
"We were off the pace and at this level it makes such a big difference. We always knew we had it in us to come back," Ashton said.
"We felt like we had everything to lose, but we're a good team and just need to keep confident with it at times."
Northampton arrived at Allianz Park in the grip of an injury crisis that had robbed them of eight senior players, numbering among them 207 caps, including Dylan Hartley, George North and Tom Wood.
But they held the lead until Ashton intervened and director of rugby Jim Mallinder was proud of the display.
"For 60 minutes we did pretty well. I was really pleased, our defence was outstanding and helped by our outstanding back row," Mallinder said.
"We looked to attack them, not run into brick walls. We're disappointed to lose, but proud of the performance. Maybe if we had our international players the final quarter would have been different."