A win over the USA should be enough to book England a semi-final spot at the T20 World Cup but Jos Buttler and Matthew Mott have plenty to prove, writes Lawrence Booth

England go into the last of their Super Eight matches on Sunday knowing that victory over the USA at Kensington Oval should be enough to guarantee a semi-final spot at the T20 World Cup - but with questions to address after a title defence that has lurched between optimism and apathy.

Friday's seven-run defeat by South Africa in St Lucia removed some of the gloss from their eight-wicket victory over West Indies two days earlier, and means that a place in the last four will have been achieved thanks to three wins over Associate nations plus a washout against Scotland, but only one victory out of three against Test-playing rivals.

Throw in their hopeless defence of the 50-over tournament in India last year, and Jos Buttler's two white-ball teams have won three World Cup games out of 10 against Test opposition. It is the kind of ratio that may yet cost him and coach Matthew Mott their jobs.

Spare a thought for the South Africans, who have won six out of six here, often from unpromising positions, yet could be dumped out of the competition in the small hours of Monday morning if they lose to West Indies in Antigua.

For England, the equation is relatively simple. Beat the USA by anything other than a narrow margin, and they will be through on net run-rate, regardless of the outcome of West Indies v South Africa, which starts several hours later.

If it rains in Barbados - and the forecast is mixed - England can progress only if West Indies beat South Africa. Even an England defeat could see them through, though that would require South Africa to hammer West Indies, and would hardly leave their semi-final opponents quaking in their boots.

First, though, England must see off an American side whose super-over victory over Pakistan during the group stages in Dallas remains the upset of the World Cup. Buttler and Co were mid-air on Friday night, travelling between St Lucia and Barbados, as West Indies showed them how to do it, knocking off the USA's total of 128 with more than nine overs to spare.

And, in fairness to England, they have so far dealt pretty well with lesser opposition, dismissing Oman for 47, then crashing 122 for five in 10 overs against Namibia.

'We've played in Barbados quite a lot the last six months,' said Harry Brook, who nearly got England over the line against South Africa. 'We know the conditions, the wind and the pitch, so hopefully we can go out there and give them a good battering.'

But they could face either India or Australia in the semi-finals, when frailties that have at times gone unpunished will come into sharper focus.

Buttler himself has not been at his best, his competition haul of 108 at 27 with a strike-rate of 131 left in the shade by his opening partner, Phil Salt, who has scored 158 at nearly 40 and with a strike-rate of 177. England lost to South Africa partly because Buttler laboured to 17 off 20 balls, leaving them 54 for three by the time he fell in the ninth over.

And that put pressure on a batting line-up weakened by the desire to include a fourth seamer (and an extra left-hander) in Sam Curran at No 7, despite a T20 batting average of just 13. Jofra Archer is at least one place too high at No 8, while England seem wedded to Mark Wood, who took three for 12 against Oman, but has none for 101 off 10 overs against everyone else.

Some of Buttler's tactics, too, have been found wanting. England erred in omitting Reece Topley from their group matches in Barbados against Scotland and Australia, and erred again by giving Will Jacks the second over against the Australians. It cost 22, and Jacks has since drifted from the side.

If things went like clockwork against West Indies, thanks in part to Salt's brutal unbeaten 87, then England's timing was awry at crucial moments against South Africa. They were too slow to work out that the best way to bowl to Quinton de Kock was to take pace off the ball, and too passive when they set about their chase.

With a World Cup semi-final theirs for the taking, this may sound like splitting hairs. But only a high-profile victory over a major team can help banish the suspicion that England have progressed in spite of their performances, rather than because of them.