
Reviewed for patch 7.3d

Favorable matchup
Reviewed for patch 7.3d
Aatrox's damage and crowd control depend on The Darkin Blade catching you with its sweet spot, the part that also knocks you airborne. Riposte blocks that swing, knock-up included, and answers with a stab; then Fiora wins the short duel by striking Vitals.
The Darkin Blade swings that catch you with the sweet spot deal more damage and knock you airborne for an instant. Riposte blocks all damage and effects for under a second and, if it absorbs an immobilizing effect, stuns instead of slowing. Save it for the swing Aatrox commits with, not the first one.
Aatrox heals with his passive and with Umbral Dash's physical vamp. Every Vital you strike deals true damage based on his max health and heals you, so the trade doesn't hinge on how much he can sustain, but on how many Vitals you reach with Lunge.
Grand Challenge reveals all four Vitals; if you hit them all, or if Aatrox dies after you hit one, you and nearby allies are healed. Save it for when World Ender is over: during his ultimate, Aatrox heals more and moves faster.
Trade when Aatrox casts The Darkin Blade and you have Riposte ready. Without the parry, leave the area of his swings and let him burn the sequence before stepping back in.
From Aatrox's side, don't open the sequence with a swing Fiora can read. Throw The Darkin Blade at the wave so she wastes Riposte, or threaten Infernal Chains to cut off her path, and go in with the sweet spot while the parry is on cooldown. Play for teamfights: World Ender is worth more among several enemies than against a Fiora who knows how to parry.

Favorable matchup
Reviewed for patch 7.3d
Aatrox needs you to stay inside the area of The Darkin Blade and Infernal Chains. Riven has three Broken Wings strikes and Valor's dash to step out of every sweet spot, stuns with Ki Burst between swings and ends the trade before Aatrox heals it back.
Every swing of The Darkin Blade has a sweet spot that deals more damage and knocks airborne. Valor moves you a short distance and gives you a shield: use it to leave that zone when you see the swing coming, not to go in.
Ki Burst stuns nearby enemies for 0.75 seconds. Between two swings of The Darkin Blade, that stun delays the next one and lets you strike with the Runic Blade charges your abilities just filled.
Umbral Dash gives Aatrox physical vamp against champions, and World Ender increases it. Riven deals her damage in bursts of ability plus attack: go in with Broken Wings, unload and get out before the trade drags on and his healing evens it out.
Trade once Aatrox has spent The Darkin Blade or Infernal Chains. If he has the full sequence, wait for him to throw it at the wave.
Aatrox against Riven has to force her to spend mobility before committing his swings. Cast Infernal Chains first: if she leaves the area with Valor or Broken Wings, she no longer has that exit for The Darkin Blade. In long fights your vamp is still the edge, so drag her into them instead of accepting her short bursts.

Execution matters
Reviewed for patch 7.3d
Aatrox is melee and has to reach you with Umbral Dash and Infernal Chains. Teemo hits him from range, runs off with Move Quick and blinds his empowered attack with Blinding Dart. It's situational: if the chains catch you and you don't leave the area, the trade becomes his.
Deathbringer Stance periodically empowers one of Aatrox's attacks to deal damage based on your max health and heal him. While blinded, his attacks miss, deal no damage and don't apply on-hit effects. Throw Blinding Dart when he has that attack ready and comes for you.
Toxic Shot adds magic damage on impact and for a few seconds after. Every attack from range while Aatrox last-hits costs him health without his swings reaching you, and Move Quick keeps you faster as long as no champion hits you.
Noxious Trap explodes when stepped on, deals damage and slows for a few seconds. Place them where Aatrox chases you or crosses toward you: a well-timed slow decides whether Umbral Dash reaches you or falls short.
Poke while Aatrox last-hits and doesn't have the empowered attack ready. If he casts Infernal Chains on you, your only goal is leaving the area; the trade starts over once the chains are on cooldown.
Against Teemo, Aatrox has to trap him with the chains, not chase him around. Cast Infernal Chains while Move Quick is on cooldown and use Umbral Dash to cover the area if he tries to escape. If he blinds you, walk away for a few seconds instead of swinging at air: your abilities still deal damage, your basic attacks don't.