The centre-back is anticipating a tough test at Wembley, despite Spurs' recent poor form but has conceded his side are confident of victory
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Roberto Di Matteo's men will take on Spurs at Wembley three days before meeting Barcelona in the first leg of their Champions League semi-final and Cahill claims the Blues are feeling confident ahead of the games.
"We're positive because of the results of late," Cahill told FATV.
"So we're confident, but there is also a realisation of how big the game [against Tottenham] is.
"It's a defining moment of the season."
Interim boss Di Matteo appears to have stepped seamlessly into the role of manager, and Cahill cited the Italian's knowledge of the Chelsea set-up as a big factor in his early success.
"The lads have taken to him well and he's been in and around the set-up for a while now, so he knows how we work and the lads know how he works," he explained.
"He's a good guy and he's a good manager as well, so it's going really well at the minute."
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The former Bolton man also added he was anticipating a stern test at Wembley despite Spurs' poor recent form with Harry Redknapp's team having won only one of their last eight Premier League matches.
"They're a hard team and Harry has turned them into a real force," he added. "It's going to be a tough game."