Saracens took a step towards the Heineken Cup quarter-finals on Friday with a 16-13 win over the Ospreys at Liberty Stadium.
As was the case last week, Saracens were able to build off the platform of a dominant line-out and held a 16-6 lead at the break as Ospreys tried in vain to find a crack in a very well-organised defence.
Indeed, the key to victory was the Sarries' impenetrable wall as Ospreys did most of the playing but struggled to breach the red-clad line with the regularity needed to force a positive result.
But the home side were determined not to give up their impressive home record without a fight and a strong second half performance - plus two yellow cards for Saracens - meant that the result was in the balance right up to the final whistle.
The back-to-back wins leave Saracens at the top of Pool Five, two points clear of Biarritz.
Ospreys fly-half Dan Biggar's penalty in the twelfth minute cancelled out Owen Farrell's earlier effort to cap a fiery opening spell that saw a few handbags fly.
The pattern continued as Farrell and Biggar traded penalties while the pushing and shoving escalated and referee Jerome Garces needed a long word with the captains to try regain control of the match that was threatening to boil over.
Ospreys had been the more enterprising side but Sarries landed the first telling blow just before the half-hour mark when Charlie Hodgson charged down his opposite number Biggar and Ernst Joubert pounced on the loose ball to score behind the sticks, allowing Farrell an easy conversion.
Ospreys dominated possession in the first half, but were unable to secure the quick ball needed to work their way around Saracens' defensive line.
Saracens took charge of the game in the closing stages of the first half as Farrell added another penalty before Ospreys loosehead Paul James was sent to the bin for repeated scrum infringements.
Much to the frustration of the players and spectators not one scrum was completed in the whole of the first half.
The home side weren't doing themselves any favours as Biggar missed an easy penalty soon after the restart. The local fans were left further frustrated when Ashley Beck was denied a try by the TMO for making a double movement to get over the line.
The hosts were however handed a lifeline when Sarries hooker Schalk Brits was also sent to the bin (for an early engagement). Brits was as much a victim of the referee's failure to communicate adequately with the players as the English club's pack's overeagerness to get the jump on their opposition.
In the end neither sin-binning resulted in a change on the scoreboard. But monsieur Garces would continue to leave his mark on the game when Kelly Brown also saw yellow for obstruction.
This time Osprey did make the most of the opportunity as Biggar floated out a beautiful pass to lock Ian Gough who wriggled over in the corner for a try. Biggar's conversion narrowed the gap to three points, setting up a grandstand finish.
But despite Farrell missing a late penalty, Ospreys were unable to find the extra try they needed for victory and now drop to third in the pool with two rounds remaining.
Pens: Biggar 2
Pens: Farrell 3
Yellow cards: James (Ospreys - 38th min - repeated scrum infringement); Brits (Saracens - 51st min - repeated scrum infringement); Brown (Saracens - 63rd min - obstruction)
Ospreys: 15 Richard Fussell, 14 Tommy Bowe, 13 Andrew Bishop, 12 Ashley Beck, 11 Shane Williams, 10 Dan Biggar, 9 Rhys Webb, 8 Jonathan Thomas, 7 Justin Tipuric (capt), 6 Ryan Jones, 5 Ian Evans, 4 Ian Gough, 3 Adam Jones, 2 Richard Hibbard, 1 Paul James.
Replacements: 16 Huw Bennett, 17 Duncan Jones, 18 Aaron Jarvis, 19 Joe Bearman, 20 George Stowers, 21 Kahn Fotuali'i, 22 Matthew Morgan, 23 Hanno Dirksen.
Saracens: 15 Alex Goode, 14 David Strettle, 13 Owen Farrell, 12 Brad Barritt, 11 Chris Wyles, 10 Charlie Hodgson, 9 Ben Spencer, 8 Ernst Joubert, 7 Jacques Burger, 6 Kelly Brown, 5 Mouritz Botha, 4 Steve Borthwick (c), 3 Matt Stevens, 2 Schalk Brits, 1 Rhys Gill.
Replacements: 16 John Smit, 17 Deon Carstens, 18 Carlos Nieto, 19 George Kruis, 20 Andy Saull, 21 Peter Stringer, 22 Adam Powell, 23 James Short.
Venue: Liberty Stadium, Swansea
Referee: Jerome Garces