Investec Champions Cup
Connacht (19) 45
Tries: Prendergast 2, Forde, Hurley-Langton, Frawley 2, Blade Cons: Gilbert 5
Saracens (19) 36
Tries: Vaenuku 2, Caluori, Tompkins, Segun, Daly Cons: Farrell 3
Connacht produced a superb attacking performance to beat Saracens in their opening Champions Cup pool game.
Captain Cian Prendergast and fly-half Ciaran Frawley scored two tries apiece as Connacht, third from bottom in the United Rugby Championship table after three games, crossed seven times against the three-time Champions Cup winners at a raucous Dexcom Stadium.
Saracens had the chance to leave with a losing bonus point, but Owen Farrell's conversion attempt after Elliot Daly's late try went wide.
The Prem club had led 12-0 early on with tries from Sione Vaenuku and Noah Caluori.
However, Connacht replied with 19 unanswered points as Prendergast, Cathal Forde and Shamus Hurley-Langton all touched down to put the hosts in front.
Hurley-Langton was sin-binned just before the break and Saracens took advantage almost immediately as Nick Tompkins dummied his way over the line from close range with the clock in the red, meaning an enthralling first half ended 19-19.
Connacht, still with 14 men, scored first in the second period when Bundee Aki set up Prendergast's second, although Saracens' response was swift as Farrell and Daly provided an easy run-in for Rotimi Segun.
Frawley's double and another from replacement Caolin Blade helped Connacht build a 14-point gap, despite centre Vaenuku scoring his second for the visitors, which proved more than enough to see out a hugely impressive victory.
A bonus point eluded Connacht, however, with teams needing to score three or more tries than their opponents to earn one in the pool stage this season.
Elsewhere in Pool Two on Saturday, La Rochelle beat South African side Lions 31-14.
Lions played the match in honour of team-mate Hyron Andrews, who died at the team's hotel on Friday aged 31, with tributes paid to the former Sale Sharks player before kick-off.
Line-ups
Connacht: Gilbert; Treacy, Forde, Aki, Jennings; Frawley, Reilly; Van Wyk, Victory, Illo, D Murray, Murphy, Prendergast (capt), Hurley-Langton, Jansen.
Replacements: De Buitlear, Bohan, Connolly, N Murray, O'Brien, Blade, Ioane, Ryan.
Saracens: Daly; Caluori, Tompkins, Vaenuku, Segun; Farrell, Williams; Mawi, George, Street, Sodeke, Tizard, Itoje (capt), Knight, Earl.
Replacements: Clarke, Brantingham, Weilbach, Isiekwe, Michelow, Barbeary, James, Burke.
Referee: Pierre Brousset (France)
Assistant referees: Luc Ramos (France) and Julien Caulier (France)
TMO: Tual Trainini (France)