London - Saracens remained on course for a double-winning campaign after reaching the semi-finals of the European Champions Cup with a 29-20 victory over English rivals Northampton.
But Sarries boss Mark McCall warned his side they would have to up their game when they faced Wasps, another English club, in the last four of Europe's premier club rugby tournament on April 23.
Despite boasting six members of England's Grand Slam-winning squad in their starting line-up, it wasn't until former Northampton wing Chris Ashton's 68th-minute try that Premiership leaders and reigning champions Saracens finally put distance between themselves and an injury-hit Saints team.
"In the first half we didn't play anywhere near to the level that we can," said McCall after the north London club's victory at their Allianz Park ground on Saturday.
"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."
Saracens captain Brad Barritt said a much-improved penalty count had been the key to their second-half revival.
"A huge problem was our discipline in the first half. It was 9-1 in penalty count (against Saracens) at half-time. Being on the back-foot played a big role," said the England centre.
"But we're a team of guys who work hard for the team. Guys like Maro Itoje all stood up. We all stuck together and galvanized."
McCall added: "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.
"We know we'll have to improve a lot of things against Wasps in the semi-finals."
Northampton's injury problems meant they were without several senior players boasting more than 200 Test caps between them, including England captain Dylan Hartley and Wales wing George North.
But they were ahead for much of the match and it wasn't until 12 minutes from time, when Ashton struck, that the game started to slip away from the Saints.
"I was really pleased, our defence was outstanding and helped by an outstanding back row," said Northampton boss Jim Mallinder.
"We looked to attack them, not run into brick walls.
"Maybe if we had our international players today would have been different."
Wasps, 13 points down midway through the second half, were indebted to Jimmy Gopperth's ice-cool conversion with the last kick of the game as they beat Exeter 25-24 in the first of Saturday's all-English quarter-finals.
Gopperth's fellow New Zealander Charles Piutau scored two tries, with former All Black Frank Halai launching their recovery when he went over in the 65th minute.
But it still needed Gopperth to land a difficult conversion from out on the touchline to seal the comeback win.
"It was edge-of-the-seats stuff. I have had a few of those in my career, and they always seem to have gone against us, so it was great to see Jimmy kick it over," said Wasps captain James Haskell.
Former England No 8 Thomas Waldrom's two tries looked to have put Exeter, second in the Premiership, on course for victory.
"It was two good sides going hammer and tongs. One had to win, and it wasn't us today," said Exeter coach Rob Baxter.
Sunday could yield another English semi-finalist when Leicester face Stade Francais.
But whatever happens at Welford Road, France will be guaranteed a representative in the last four when defending champions Toulon, bidding for an unprecedented fourth straight European crown, face Racing 92 in a match that will see New Zealand World Cup-winners Ma'a Nonu and Dan Carter on opposite sides.