A moment of opportunism from Gareth Cooper and a brief glimpse of Iestyn Harris magic to create Martyn Williams's try were the only highlights in an otherwise uninspired performance.

Tonga outscored Wales three tries to two but crucially Pierre Hola missed four kicks at goal as Wales clung on against the South Sea island minnows for their second win of the Rugby World Cup.

Stephen Jones got Wales on their way with a penalty within just four minutes and put Wales ahead again with a second some eight minutes later after Hola had levelled the scores, and when Cooper scampered over from the tail of the scrum some 15 yards out, Wales looked to be on their way.

A mistake in defence by full-back Rhys Williams, who failed to collect Hola's kick ahead in the wet conditions mistake, allowed the Tongan outside-half to pick up and score to bring the scores back to within a point just four minutes later.

Neither side were able to take a grip on the game in the wet conditions though two more Jones penalties either side of half-time put Wales 17-10 ahead. Nevertheless Wales struggled and after Jones missed three attempts at goal, it was left to flanker Martyn Williams to slot over a drop goal just three minutes after taking the field as a replacement.

Wales could not shake off Tonga who hit back with a try from captain Benhur Kivalu and even after Williams had latched onto Harris's try for Wales second, the south sea islanders pulled one back again through prop Heamani Lavaka.

That set up a tense last two minutes for Wales and had Sukanaivalu Hufanga caught Hola's pass he looked destined to score a try that would have levelled the scores. However, Wales held out and now face Italy for the game that is likely to decide who join the All Blacks in the last eight.