Paris - Defending champions Saracens battled back from a 10-point deficit to edge Ospreys 36-34 in a European Cup thriller on Saturday as Clermont's French international flyhalf Camille Lopez suffered a sickening broken left ankle.
Saracens, chasing a third successive title, were 17-7 down to their Welsh guests but despite running in five tries and staying ahead for the majority of the second period, Ospreys kept hitting back.
The Welshmen finished with four tries and two bonus points although Saracens still sit top of Pool 2 with 10 points having started their campaign with a 57-13 thumping of Northampton last week.
Chris Wyles, Schalk Brits, Liam Williams and replacement centre Nick Tompkins scored for Saracens.
Their other score was a penalty try awarded for a deliberate knock-on by the impressive Dan Evans who was sin-binned for his troubles.
In his absence, Ospreys conceded 12 points which proved decisive in the final analysis.
"We showed some good resilience. To be down 10 points and come back before half-time felt like a good position to be in," said Saracens director of rugby Mark McCall.
"Give Ospreys all the credit. They played phenomenally tonight."
Earlier in the same pool, French international Lopez was stretchered off with a broken left ankle in Clermont's 24-7 win over Northampton.
The 28-year-old Lopez was writhing in agony after a clash with Northampton's Nick Groom in the first half and has been ruled out for five months.
The 16-times capped Lopez will miss the autumn internationals as well as the Six Nations.
"When I saw Camille's injury, it left me cold," said Clermont scrumhalf Morgan Parra.
"We know how important he is on the pitch as a player and within the squad."
Two-time champions Leicester Tigers demolished Castres 54-29 at Welford Road having lost their Pool 4 opener 22-18 at Racing 92.
Leicester ran in seven tries, including a hat-trick by Tongan winger Telusa Vieanu, taking a bonus point in the process.
However, Castres, who were 35-3 down at the interval, assured a losing bonus point with four tries in the second half -- three of which came from veteran Samoan winger David Smith.
"To take six points out of those first two games is important and puts you in a really good place from where you can probably control your own destiny," said Leicester coach Matt O'Connor.
Munster, also two-time champions, edged Racing 14-7 in a game which was deadlocked at 0-0 on the hour mark.
Scrumhalf Conor Murray and replacement winger Andrew Conway crossed for two converted tries before the French side grabbed a losing bonus point with a late try from lock Leone Nakarawa.
French flyhalf Francois Trinh-Duc kicked a last-minute penalty as three-time champions Toulon escaped humiliation by edging out Italian minnows Treviso 30-29.
Treviso thought they had claimed a famous victory when Irish flyhalf Ian McKinley, wearing trademark goggles to protect the eye injury he suffered as a student seven years ago, had knocked over his fifth penalty of the day in the 79th minute.
But Toulon, who had struggled to a 21-20 win against Scarlets last weekend, powered forward on one last surge which saw a tiring Treviso penalised at a ruck.
Trinh-Duc kept cool to kick the winning points for a personal match tally of 20 points.
Treviso had lost their opening game 23-0 at Bath but matched their superstar visitors throughout the game, scoring two first-half tries from Juan Ignacio Brex and Edoardo Gori for a 14-13 advantage at the break.
Semi Radradra crossed for Toulon in the opening period with fellow Fijian winger Josua Tusiova adding a second try early in the second period.
Bath lead Pool 5 after seeing off Scarlets 18-13 in Wales on Friday night.
Three-time champions Leinster enjoyed a 34-18 win at Glasgow, their second successive bonus point win in Pool 3.
Cian Healy scored two of Leinster's four tries as Glasgow slumped to a second loss.