Needing just a point to safeguard their passage to the knockout stages, the English champions laboured and had to erase a 17-13 half-time deficit before pulling clear in the second half. Mouritz Botha and David Strettle both touched down for Saracens and Owen Farrell kicked 16 points to set up a home quarter-final against Clermont.
Mark McCall's men are now the only English side left in contention in Europe's top tournament, but they will know that they will have to raise their game several notches when the knockout stages start in early April. They came into the match hoping for a bonus-point win to seal a home quarter-final but Treviso, unbeaten in their previous two pool games at home, were in no mood to lie down.
"I thought we were miles off where we normally are, and especially in the first half," McCall said. "The intensity wasn't there, and neither were the standards we normally expect. I was pretty disappointed at half-time. But then in the second half we dug in, and I have nothing but admiration for the way we got the job done."
Farrell struck first blood with a penalty after 10 minutes but they were rocked on their heels when Robert Barbieri dotted down for the opening try moments later, which was converted by Kris Burton. Sarries hit back with a try of their own, Botha barging over from close range after the Saracens forwards had pummelled the home pack. Farrell made no mistake with the conversion but again the visitors were found wanting in defence, and they conceded a second try more or less off the restart.
Barbieri was once again instrumental, punching a massive hole in the Saracens midfield before the ball was spun wide to Tommaso Iannone, who touched down in the corner. Burton added the conversion and then an opportunistic drop-goal to leave Treviso with a 17-10 lead 25 minutes in.
With their pool rivals Biarritz by this stage 17-0 up against the Ospreys, the wheels were in serious danger of coming off. Farrell reduced the deficit to four points just before the break, but at half-time Saracens' prospects were still looking touch and go.
Thirty seconds into the second half, however, the game took a seismic swing when Strettle blocked an attempted clearance and strolled his way over the line for a gift-wrapped try. Farrell converted and then traded penalties with Burton to keep his side just ahead, at 23-20. The Treviso scrum-half, Tobias Botes, then looked to have scored a perfectly good try, but it was mysteriously overruled by the stand-in referee, John Carvill, who had replaced the injured Alain Rolland in the first half.
Saracens, at this stage, were hanging on by their fingernails, but the accuracy of Farrell allowed them to keep the scoreboard ticking over. Any hopes they may have had of gaining the bonus point they needed were ended in the final quarter by Treviso's attempt to find a way back into the game.
Scorers: Benetton Treviso: Tries: Barbieri, Iannone; Conversions: Burton 2; Penalty Burton; Drop goals Burton. Saracens: Tries: Botha, Strettle; Conversions: Farrell 2; Penalties: Farrell 4.
Benetton Treviso: L Nitoglia; T Iannone (A Di Bernardo, 75), T Benvenuti, A Sgarbi, B Williams; K Burton, T Botes (F Semenzato, 58); M Rizzo, L Ghiraldini, L Cittadini, A Pavanello (G Padro, 36), C van Zyl, B Vermaak, A Zanni (capt), R Barbieri (M Filipucci, 58).
Saracens: A Goode; D Strettle, O Farrell, B Barritt, J Short (C Wyles, 51); C Hodgson, P Stringer (B Spencer, 72); M Stevens, J Smit (J George, 60), C Nieto (R Gill, 45), S Borthwick (capt; H Smith, 32), M Botha, K Brown (J Melck, 32), A Saull, J Wray.
Referee: A Rolland (Ireland) (J Carvill, Ireland, h-t).