Wayne Barnes is the man in the middle today. He blows his whistle, Quins fly-half Nick Evans kicks the ball long and Billy Vunipola gathers. The second semi-final is underway.

BBC Sport's Chris Osborne at Allianz Park: "Flying the flag. No expense spared by Saracens. Flags have been spread like crops across the seats at Allianz Park. Northampton handed out pieces of card last night - Sarries are just showing off really."

Now, we have a late change to the Saracens team to tell you about. Lock Mouritz Botha misses out, so Alistair Hargreaves steps up from the bench. Eoin Sheriff fills the void left by Hargreaves among the replacements.

Image source, Getty Images

Saracens and ex-England captain Steve Borthwick, who is set to retire from the sport this summer, will make his 264th Premiership appearance this afternoon, which is more than any other player has managed in the English top flight.

Meanwhile, number eight Nick Easter will set a new Harlequins record by playing his 233rd game for the club.

Saracens have a pretty tasty recent record against Harlequins, having won each of their previous four meetings and only lost one of the last 10.

This season, Mark McCall's side beat Quins 22-12 at The Stoop in September, before easing to a comfortable 39-17 victory in front of a record-breaking crowd at Wembley in March.

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While their hosts were able to rest their big-hitters last week, Harlequins were not afforded the same luxury, with their spot in the play-offs still in the balance.

They have made just one change to the side that edged past Bath, with Luke Wallace recovering from injury to start at blindside flanker in place of Maurie Fa'asavalu.

It means they will be leaning on their England contingent of club and country captain Chris Robshaw, prop Joe Marler, scrum-half Danny Care and full-back Mike Brown, the Premiership's player of the year.

Harlequins: Brown; Williams, Molenaar, Turner-Hall, Monye; Evans, Care; Marler, Ward, Sinckler, Matthews, Robson, Wallace, Robshaw (capt), Easter.

Replacements: Buchanan, Lambert, Collier, Kennedy, Guest, Dickson, Botica, Smith.

Saracens have changed their entire starting XV from that which lost 31-27 at Leicester last weekend.

Among the returnees is England fly-half Owen Farrell, who is Sarries' top points-scorer this season.

Saracens: Goode; Ashton, Bosch, Barritt, Strettle; Farrell, De Kock; M. Vunipola, Brits, Stevens, Borthwick (capt), Botha, Brown, Burger, B. Vunipola.

Replacements: George, Barrington, Johnston, Hargreaves, Wray, Wigglesworth, Hodgson, Wyles.

BBC Sport's Chris Osborne at Allianz Park: "The clouds are rolling in amid forecasts of storms. Plenty of opportunities for cliches about electric atmospheres and thunderous performances. But you'll get none of that from us. The air is thick, muggy and rank with anticipation. If today's game is anywhere near as good as last night's, we're in for a treat."

If "preparation is the key to success", as Alexander Graham Bell, external once proclaimed, then you'd have thought today's second Premiership play-off semi-final between Saracens and Harlequins at Allianz Park was a foregone conclusion.

While Quins were made to wait until the very final kick of the final round of fixtures to be sure of a spot in the domestic end-of-season shoot-out, 50 days have already passed since Mark McCall's side confirmed their top-four berth by beating London Wasps at the end of March.

But if last night's epic between East Midlands rivals Northampton and Leicester taught us anything, it's that nothing is a given in sport. And, as we prepare for the first Tigerless Twickenham showpiece for 10 years, the only question that remains is who'll be joining Saints in the final on Saturday, 31 May.