When you’re watching opera in concert, there’s nothing quite like those moments when Wagner decides to shift the orchestra up a gear, thickening up the string sound, blending in woodwind and brass and assaulting you with a leitmotif that he’s been carefully setting you up for. In last night’s performance of Act III of Tristan und Isolde, Valery Gergiev and the Mariinsky Theatre Orchestra showed their mastery of such moments: time after time, the Barcelona Auditori’s generously sized Sala Pau Casals was filled to the rafters with the waves of sound.

Tristan und Isolde has one of opera’s most demanding tenor roles: even without the first two Acts, the strength and stamina demanded are considerable – even more so when you’re having to compete with the Mariinsky in full cry without the protection of an orchestra pit. Act III is especially tricky because you have to produce all this volume while in the character of a dying man. Mikhail Vekua showed himself to be fully up to the challenge: his voice retained body, clarity and balance, the words of nostalgic regret coming out heartfelt.

Vekua was helped by two high quality partners. The act’s opening belonged to Evgeny Nikitin’s Kurwenal: in a strong smooth voice, Nikitin gave a compelling portrayal of the ever-loyal retainer. Towering above the rest of the cast, Eva-Maria Westbroek gave us an Isolde of stature to match: her voice has so much in reserve that she was able to throw emotion into every line of the Liebestod without ever going shrill at the top or losing intensity at the bottom. Within the constraints of a limited part as Brangäne, Yulia Matochkina impressed with similar intensity and a creamy mezzo.

But the performance belonged to the orchestra. The shepherd’s call on cor anglais was the first of a series of virtuosic woodwind solos, there was glorious string timbre in every register, lustrous brass sound and hardly an error. The big entrances – when Isolde’s ship is first sighted, and when she herself enters – were sensational. The final resolution of the famous suspended Tristan chord was thoroughly satisfying.

The Tristan und Isolde formed the second half of the concert, having been preceded by a somewhat mixed programme. We kicked off with Wagner’s sunrise over the Scheldt in the Prelude to Act 1 of Lohengrin: the high strings seemed rushed at first, but things improved as the music thickened out into a regal fanfare. Next was the chance of George Li, the silver medallist in the 2015 Tchaikovsky competition, to take his place in the limelight as soloist in Liszt’s Piano Concerto no. 2 in E flat major. Li impressed from the outset with a beautifully judged glissando which softened as it merged into the following phrase, and he showed himself at his best in the cantabile sections, giving beautiful contours to the phrasing. High trills were sustained with evenness, and Li held his own in the big crash chord passages. This was Gergiev doing what he does best: bringing top class young talent to the attention of audiences at the world's big stages.

The final scene of Salome, which closed the half, was less than successful, other than as a warm-up passage for Westbroek. The problem is that Strauss’ orchestration is so rich and thickly layered that it’s all too easy for the soprano to be swamped – even a soprano with as much power as Westbroek – and that’s what happened here. To often, I failed to hear Westbroek properly when her face was pointed away from my direction. Balance apart, there was plenty of drama in her portrayal, with a particularly intense moment at the beginning of the closing section “Ah! Ich habe deinen Mund geküsst, Jochanaan”.

Even great conductors have their bad days, and one of Gergiev’s came in a performance of Tristan in the Liceu three years ago, when things did not go well with a replacement tenor, brought in when Vekua was indisposed. Gergiev, I was told, was extremely keen to put things right with the Barcelona public, pairing a fully fit Vekua with a pair of performers in Nikitin and Westbroek who amply justified their star billing. Mission accomplished.