Only once in their immeasurably long lives had Loki bested Thor in single combat, and the one time he did, there was no one around to witness the glorious event.
It had been a bright, hot day, and the two brothers were training together in one of the many sparring halls situated within the palace. Both had begun in full armour, but the heat had proved too much for it to be worth keeping it on, and they'd stripped down to nothing but their baggy trousers. The sweat gleamed on their bare skin and they moved almost too fast for the eye to follow. Indeed, if a mortal had been watching their spar, they might well have missed all but every third blow or so. Loki wielded a staff with a blunt end and a spiral to the wood at the top, and Thor used Mjolnir. Loki's weapon had been specially wrought to withstand Mjolnir's wrath and would not shatter, even under the heaviest of blows.
Each knew the other's fighting style as well as they knew their own, and at that age had been almost equally matched. They were nineteen and knew each other better than anyone. Loki could anticipate every move his brother was going to make –it was textbook stuff he threw at the younger sibling- and Loki blocked every swing of the mighty hammer. Thor, in general, took the offensive, but every so often Loki got a go at something other than defence. When these rare occurrences came to pass, Thor often gained a couple more scratches, cuts, scrapes, and bruises than he usually had upon leaving. Loki often walked away with little more than a few light bruises and aching muscles from constantly deflecting aggressive blows.
Loki could recite Thor's routine move by move and imitate it to perfection (including the grunts and growls) if he was handed a hammer and told to entertain a crowd without magic. Thanks to that, the only way to add interest to their otherwise dull routine was for Loki to do something unexpected. This meant that every other day he needed to learn new moves in both offense and defence. It also meant Loki had a wider range and better ability than his brother. It was thus that Loki finally managed to out manoeuvre his stronger, taller, more irritating brother.
On that extensively hot day when they were younger, they'd taken a five minute respite to cool down and have a drink, and Loki got the opportunity to begin the spar for a change. He had already gotten a few hits in and Thor was bruising rapidly. He felt a rush of smugness, and once he'd taken his fill of water he picked up his staff again and took up a steady stance. Thor was facing the other way, and Loki had the element of surprise in his side. With light, quick, and silent steps Loki advanced towards the blond, and swung his weapon.
The five and a half foot long staff of reinforced wood caught Thor unaware in the side of his head. Loki smiled, his teeth showing, as Thor let out a yell and fell into a bench. He stuck out his hand, but before his trusty hammer could reach him, Loki struck again, this time catching Thor's waist. The latter stumbled and fumbled as the hammer flew into his hand. Loki grinned, the smile twisting his pointed features.
Thor turned and swung. It was a move made from desperation and as a result it was sloppy and Loki blocked it easily. He attacked again whilst Thor's guard was down, and he was spun into the centre if the room, pursued by the attacker. Loki, it transpired, did very well when his brother was unable to fully steady himself or truly get his bearings. The younger brother landed blow after blow and Thor tried his best to deflect them but managed little against the controlled, well timed, and constant swings. Eventually he got control of Mjolnir, but he was ill-prepared for the flurry of strikes Loki used against him. It was where his advanced training finally failed him; a surprise attack using moved he'd never seen before and against his younger brother.
For the first time ever, Thor found himself at a disadvantage and at the hands of someone he considered below him skill-wise. Surprise was the pervading emotion Thor felt as he struggled to fend off well-executed and perfectly aimed strikes. Loki saw every opening no matter how small in Thor's attempt at defence, and slipped past his guard after every feint. He threw odd combinations of new moves to keep Thor on his feet, and experimented in different ways of pulling them off. It was by far the best session Loki had had in a long time.
"Come on, brother! Surely you can do better than that?" He goaded as he ducked a blow and brought his staff crashing into the side of his face as Thor stumbled from the continued motion of Mjolnir. The blow sent Thor reeling backwards and Loki descended upon him, twirling the staff above his head and bringing it arching sideways down to Thor's left side, then his right before aiming it at his head, but Thor anticipated and ducked, catching Loki with a thrust to the chest with Mjolnir.
Loki was thrown backwards, but rather than landing on his behind he stretched out his lithe fame and flipped, hands momentarily touching the ground as he propelled himself over onto his feet using the momentum from the hit, and coming to stop with his arms raised to the side ever so slightly and his back bent forwards. He gave himself half a second to recover before he launched another sequence against Thor. Thor was now collected and more prepared, resulting in fewer of Loki's blows landing, but he was still hard-pressed to get a single retaliatory blow in.
Loki dropped back, one hand flat on the floor, both legs bent and his torso and knees parallel to the ground to dodge a frustrated throw of Mjolnir. He easily launched himself back to his feet and whilst Thor stood with one hand open for the hammer and the other arum up in a guard, Loki brought his full force down upon his brother. Every scorn, mocking tone, lie, taunt, and mocking laugh ever aimed at Loki over the years sprang into his head and lent him strength to floor his older brother with three well dispatched hits. Feint to the head to bring the hands up, knees, head, and centre chest to knock him down.
It seemed to happen in slow motion and Mjolnir dropped to the ground with a dull thud, still feet away as its owner staggered back once, twice, before falling down, hurled backwards by the force the last jab was received with. Loki took two running strides before leaping up and across the five feet that separated them. With a wild battle cry and no emotions but smugness, pride and want for revenge for all the times he'd been beaten in this over the years, Loki brought the staff down with a resounding BANG!
Thor watched in dazed horror as Loki all but flew at him and found himself incapable of doing anything but squeezing his eyes shut before what he knew would be the final hit. He heard a yell from Loki and the stick whistling through the air. He brought up his arms up to protect his face but that seemed to be as much as he could move. He felt feet land heavily either side of his waist and felt the stick land millimetres from his left ear. He almost let out a very un-manly scream, but held it in. The resounding BANG of wood hitting wood at high speed and force nearly deafened the Prince of Asgard, and in a mess of emotion his eyes flew open to see cold green eyes inches away from his own. Loki's grinning face was right above his, and with a feeling of both relief and release he let out a huge breath and lay panting on the floor. He closed his eyes, and felt the adrenaline begin to seep out of him, his hands falling limply to his sides.
The feet at his sides disappeared, along with the threat of the stick that had come down by his head. Loki waited until Thor's eyes opened once more and offered a hand. Thor took it and allowed himself to be pulled meekly to his feet, his face red from exertion and humiliation. Loki pulled him close and in his good ear whispered two words he has never uttered to his brother since.
"I win."
His tone held all his snarky smugness and conveyed the pride he felt. He twirled his staff between his fingers, rested it on his shoulder and sauntered out the room, leaving an almost dumbstruck and very impressed (though he'd never admit it) Thor, stood alone in the middle of an empty hall.