“It's a tiny, tiny little concerto with a tiny, tiny little Scherzo”, Johannes Brahms coyly remarked in his 1877 of his newly composed Piano Concerto No 2. This, despite it being at the time probably the largest ever piano concerto in terms of size, scope and ambition. Still, why trumpet this when all would be revealed when the music began? And in terms of both this concerto's scale, and Brahms's low-key description of it, it was the most perfect of pieces with which to open Thursday night's prom.

Oliver Knussen © BBC | Mark Allan
Oliver Knussen
© BBC | Mark Allan
First, there was the fact that the evening's other draw was the mature work of another master, Reinbert de Leeuw's The Night Wanderer in its UK premiere. Even more than that, though, it was the whole evening's mood, which had a Brahmsian, “let the music do the talking” groundedness to it from the moment pianist Peter Serkin and conductor Oliver Knussen took to the stage. First came Serkin, looking for all the world in his three-piece suit and glossy red tie as though he'd just slipped out from delivering a Yale lecture, quietly acknowledging the audience's raptly respectful applause from as far back from behind his piano stool as could be physically managed when there was an orchestra taking up the rear space. “Thank you, although actually it's not about me”, his body language said, and Knussen mirrored this as he gently applauded his friend from behind the piano lid, before unostentatiously settling himself on the stool from which he was to conduct.

Still, the evening was of course all about the artistry of these two great men, beginning with the no-frills perfection of Knussen's programming itself; no overture warm-up, but simply the Brahms then the De Leeuw, the two works so complementary in both duration and romantic thought that had they been placed on some mystical set of musical scales they would have come to rest at a precise 180-degree balance to each other.

In advance, the programme had also intrigued; what connections might such contemporary luminaries such as Knussen and Serkin draw between these two works? Would they perhaps present a laser-like, modernist Brahms concerto, taunt of thought and lucidly fluorescent of texture, pointing us towards his influence on the Second Viennese School which had in turn played such a role in the development of avante-garde De Leeuw?

The answer, however, was no. Instead, Serkin's first piano call as it rose from underneath the opening horn was of such an extreme, soft-toned pianissimo that initially you struggled to hear him at all, followed by a cadenza that was more reflective than spiky, generous but never soupy on the pedal, and these few minutes set the tone for the whole. As the concerto unfolded in such rich, soft, expansive but utterly poised style that it seemed to contain a universe of thought, this felt like a reading from men who had lived and accepted, and the BBC Symphony Orchestra were with them heart and soul. Serkin himself was riveting, not least for the way in which his vibrato action on the keys frequently culminated, as his hand came up, with a contained-but-emotionally-potent little fisted shake above the keyboard. Likewise, the bond between him and Knussen was electrifying to an extent that sometimes had you actually holding your breath; take the soft, transitional passage just before the first movement's recapitulation where, as tonality momentarily slipped its moorings into almost atonal waters, Serkin and Knussen actually physically leant in towards each other over the side of the keyboard, providing a steady visual anchor in this sudden harmonic sea of uncertainty. The power of it almost can't be quantified in words.

Then came De Leeuw's tense symphonic poem on the horrors of death and murder, with its gargantuan scoring for 100-strong onstage orchestra, off-stage orchestra and tape recording. Here there was more obvious spikiness – dogs barked and bells tolled on tape, percussion pounded, brass shrieked antiphonally at each other from opposite ends of the Royal Albert Hall like Gabrieli on acid – but overall the effect mirrored the Brahms in its overwhelmingly soft, expansive musical depiction of desolate silence. In fact, this was a work that really took Romanticism to a kind of modernist apotheosis, and Knussen's direction of the BBC Symphony Orchestra – who themselves were on truly magisterial form – matched this, his fluid gestures having grown in size from the relative concision of his Brahms conducting. The work's only slight jarring point was the taped recital of the Hölderlein poem from which the piece is named; dark, fruity and clear, the voice didn't chime with the work's otherwise otherworldly sound world, and I found myself yearning for live, perhaps even Sprechstimme tones.

So, there it was. When seventy seven year old De Leeuw took to the stage to embrace Knussen, the emotiveness reached fever pitch, and as we left the Royal Albert Hall it was with a sense of awed surprise. For, on the evening the modernists had came to the Proms, they had brought the house down with ….Romanticism.