Leinster and Northampton Saints face a five-day turnaround between the final two rounds of the Heineken Cup pool stages.
The two sides, who recorded away wins over each other this month, are competing for top spot in Pool 1 but the hopes of the pool winner securing a home quarter-final have been dealt a blow.
Leinster must travel to France to face Castres on Sunday, 12 January before hosting the Ospreys at the RDS the following Friday, while Northampton face a similar schedule away at Ospreys and at home to Castres.
The Anglo-French clashes between Harlequins and Clermont Auvergne, and Toulouse and Saracens, are the pick of the ties in round five.
Quins welcome the Top 14 heavyweights to the Twickenham Stoop on Saturday, 11 January, with Sarries travelling to the Stade Ernest Wallon the following day.
The final round of fixtures includes the meeting of Pool 5 contenders Ulster and Leicester Tigers at Welford Road on Saturday, January 18, with Munster hosting Edinburgh in the early kick-off on Sunday.
The pool winners will be ranked by reference to the number of match points earned. The top four will be rewarded with home quarter-finals, with the remaining two pool winners and two best-placed runners-up completing the last-eight.
ROUND FIVE FIXTURES
Friday, 10 January
Ulster v Montpellier, 8pm
Racing Metro v Scarlets, 8pm
Saturday, 11 January
Harlequins v Clermont Auvergne, 1.35pm
Benetton Treviso v Leicester Tigers, 1.35pm
Exeter Chiefs v Glasgow Warriors, 3pm
Toulon v Cardiff Blues, 3.40pm
Connacht v Zebre, 3.40pm
Edinburgh v Perpignan, 6pm
Gloucester v Munster, 6pm
Sunday, 12 January
Ospreys v Northampton Saints, 12.45pm
Castres Olympique v Leinster, 12.45pm
Toulouse v Saracens, 3pm