Leigh Halfpenny scored 15 points on his return to Toulon, but his penalty miss 10 minutes from the end cost the Welsh region, who had lost their previous nine away matches in Europe, victory in a match that showed their pre‑tournament billing as one of the likely quarter-finalists was not misplaced.

The Scarlets were 15 points down in as many minutes, caught out by Toulon’s willingness to move the ball, but there were periods in the second half when the team that won the Champions Cup three times in a row this decade was hanging on before recovering to win 21-20.

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The Scarlets, who had left with nothing on their two previous visits to Toulon, secured a bonus point but they face Bath in Llanelli on Friday, five days after a physically gruelling encounter. “Maybe we were given the short turnaround because we were not expected to go through,” their captain, Scott Williams, said. “We showed we are a good enough team to progress and, despite the slow start, it was a game we could have won.”

Toulon’s first try came from a Scarlets mistake after five minutes: a loose tap back from a lineout on their 10-metre line saw Aled Davies pass when he should have held on to the ball and Eric Escande intercepted. The second resulted from a sense of adventure for which Toulon are not renowned. A quick drop out gave Josua Tuisova space and the wing galloped through Halfpenny’s challenge to set up Guilherme Guirado.

Anthony Belleau’s second penalty gave Toulon an 18-point lead but the Scarlets worked their way back. Halfpenny, on his first return to Toulon since leaving last season, made up for an earlier penalty miss with a 20-metre kick and when Steff Evans’s break resulted in a yellow card for Hugo Bonneval after the full-back tackled Davies without the ball, Halfpenny trimmed the home side’s interval lead to 15 points.

The Scarlets started the second half as Toulon had the first. Rhys Patchell’s chip into space that would have been covered by Bonneval saw Chris Ashton beaten by the bounce which favoured Johnny McNicholl. Eight minutes later Halfpenny crossed the line after using Tadgh Beirne as a decoy.

The Scarlets were showing the form that took them to last season’s Pro12 title and took the lead on the hour through Halfpenny’s boot. Toulon regained the advantage through François Trinh‑Duc after Beirne entered a ruck from the side and both teams pressed for victory, Halfpenny missing from wide 10 minutes from the end.

Toulon’s Argentina back-rower Fecunda Isa was denied a try with what would have been the last play for a double movement after Sebastien Tillous-Borde had been held up over the line. The Scarlets, who by then had Beirne in the sin-bin, ran the ball from their own line before accepting the bonus point.

Elsewhere, Munster were relieved to leave Castres with a 17-17 draw while Ospreys went down 26-21 against Clermont Auvergne in a game in which Clermont held a 20-7 half‑time lead. A disappointing result was compounded by a serious-looking injury to the Wales international flanker Dan Baker.