Farrell and company arrived in last season's competition as Premiership champions, but it all unravelled when French heavyweights Clermont Auvergne crushed them 22-3 at Vicarage Road.
"We've come on leaps and bounds as a team since that defeat to Clermont," said Farrell, whose fly-half battle with Jonny Wilkinson could decide the outcome. "They came over and we were never really in the game. We had won the Premiership the year before, but that put things into perspective for us."
He added: "This is a tournament that is the best of the best, and to be involved in a semi-final against a great team like Toulon is fantastic. We are all looking forward to the challenge Toulon set."
Saracens qualified from a tough group that featured Munster and big-spending French club Racing Metro, with home and away successes against Racing underpinning their push for the quarter-finals that resulted in a Twickenham demolition of Ulster.
Flanker Will Fraser, though, who was a try-scorer in the Ulster clash, misses out because of a shoulder injury suffered during last weekend's Premiership defeat against Gloucester.
Scotland captain Kelly Brown returns to the back row, packing down alongside Jackson Wray and Ernst Joubert, while props Mako Vunipola and Matt Stevens - who are both believed to be on British and Irish Lions head coach Warren Gatland's selection rader - have a final chance to impress ahead of next Tuesday's Lions squad announcement.
Wilkinson, meanwhile, features in a star-studded Toulon side, although there is no place for England international flanker Steffon Armitage.
Armitage is on the bench - coach Bernard Laporte has preferred a back row of Danie Rossouw, Juan Martin Fernandez Lobbe and Chris Masoe - with the replacements also including Wales prop Gethin Jenkins, plus France international backs Maxime Mermoz and Frederic Michalak.
Wilkinson and Lobbe are among seven former Premiership players in the Toulon team, being joined by full-back Delon Armitage, props Andrew Sheridan and Carl Hayman, hooker Sebastien Bruno and lock Nick Kennedy.
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