Cardiff - Pakistan may be the underdogs heading into a Champions Trophy semi-final with England in Cardiff on Wednesday, but coach Mickey Arthur believes their "best game" could see his side cause a major upset. 

England were the only team to exit the group stage unbeaten, having won all three of their matches.

Pakistan, by contrast, suffered a 124-run thrashing by title-holders and arch-rivals India in their opener before, just days later, upsetting the ranking in a bottom versus top contest by beating South Africa. 

It then needed an "ugly" win over Sri Lanka in Cardiff on Monday to take Pakistan into the last four of a tournament featuring the world's top eight one-day international sides. 

Pakistan, chasing a seemingly modest target of 237 against Sri Lanka, were facing defeat and tournament elimination at 137 for six. 

But Pakistan captain Sarfraz Ahmed, dropped twice, struck a superb 61 not out and received excellent support from Mohammad Amir (28 not out) in an unbroken eighth-wicket stand of 75 that sealed a tense three-wicket victory. 

"When you win ugly, you learn a lot about the team," said Arthur, a former coach of both his native South Africa and Australia. "So I guess it gives a lot of confidence." 

But he said a semi-final could not be the summit of Pakistan's ambitions, with an all Asian final against either India or Bangladesh, who play Thursday, in their sights. 

"The last thing I want is for us to go away now thinking that we got to a semi-final, we're okay, we've achieved, because that would be a cop-out in my mind," he said. 

Pakistan certainly made had work of the chase after their bowlers had held Sri Lanka to 236 all out, with pacemen Junaid Khan (three for 40) and Hasan Ali (three for 43) doing the damage. 

"Sometimes that's our strength, our unpredictability," said Arthur. "It's just we do sometimes make it difficult for ourselves." 

Well though Sarfraz batted, he needed the support of Amir, primarily a left-arm fast bowler. 

Now back in the side after a ban and jail term in a British prison for spot-fixing, Amir is becoming an ever more important member of the Pakistan team and Arthur said: "He showed what we know he can do with the bat yesterday (Monday)." 

Sarfraz, meanwhile, probably wishes he could bat at Cardiff all the time, with the wicket-keeper having made 90 on the same ground last year as Pakistan chased down a target of 303 against England in a four-wicket win that prevented a 5-0 ODI series whitewash. 

"We win that time, we chased 300, so definitely our confidence is very high playing in Cardiff," Sarfraz said after Monday's thrilling triumph. 

However, England captain Eoin Morgan played down the importance of last year's result by saying on Tuesday: "I think the impact of that series was 4-1. 

"Obviously, on their day, they (Pakistan) can beat any team in the world...They're very dangerous." 

But Arthur was in no doubt of England's quality, saying: "They're a really good one-day unit with no apparent weaknesses." 

England, however, are set to drop opener Jason Roy, whose latest low score in the victory over Australia last time out means he has now managed just 51 runs in eight ODI innings this season, and replace him with Jonny Bairstow. 

Morgan, whose 87 together with Ben Stokes's 102 not out saw England recover from a top-order collapse against Australia, would not confirm England's XI on Tuesday. 

But he signalled a change by saying: "We're getting to the business end of the tournament and we need to produce results." 

Roy apart, England appear to have all bases covered as they go in search of their first major ODI title, with a long batting order and a balanced attack led by fast bowler Mark Wood.

"I think we've started brilliantly," said Morgan. "I hope we've not played our best cricket yet."