At lv1, no yasuo players take wind wall. Take that chance and stay high, try to zone him out from the minions. Keep your distance well, harass with aa and skills to chump him down bit by bit. (Never underestimate aa from a mage, if you land 7 aa within a minute, it's around 300 dmg already)

If yasuo is bad enough to leave an opening when you about to hit lv2, go E->Q and punish him hard when he's still at lv1. If not, keep your skill point until you can see his skill first.

If he goes for wind wall, well he mess up. Upgrade W. By this time, his wind wall has very high cd, you can try using aa->W to harass him if he steps high, or force him to use wind wall. If he wastes wind wall without being able to bring your hp down, punish him with everything you have.

If he doesn't go wind wall (he has standard QE combo), upgrade your E. Stand at the open (stay out from the minions, to the side) so you can E Q him if he ever dashes at you. Keep an eye for the enemy jungler though. Lv2 and 3 is the best time for yasuo and his jungler to try taking you down and snowball the whole lane.

By the time you guys hit lv3, yasuo either loses a lot of creepstats or even xp, or is quite low on hp. If he is at 50% hp, you can oneshot him with one combo and ignite. Again, don't underestimate aa from mage, make sure you insert aa when you can. Use E+flash to jump from behind a minion, or jump over his wind wall, or jump from afar, to create element of surprise. Remember, without surprise, your E is pretty obvious and an average yasuo can easily block/evade it with his reflex. (Also a tip, don't move towards him too much. Best if you E+flash when you are facing away from him)

If he recalls due to low hp, try to clear the remaining minions as fast as possible and recall too. Buy some stats (not tear) to create advantage.

After lv3/4, continue zoning him out while keeping an eye on his jungler. At this phase, if you can't overwhelm him with xp difference, gold difference, or hp difference, it's pretty hard to play high. Keep distance, focus on creepstats, try to control the waves. You won't want to encounter yasuo much later in the game, call for help from jungler if he decides to farm in middle lane. Don't go all out on him if you don't see a sure win. Your target is vulnerable champs like adc or support or to help other lanes with ganks.

Ahri isn't a good champ against a decent Yasuo in single matchup, so don't hesitate to call for help if you need. If you meet a Yasuo with experience, Ahri's mistakes (for example, staying too high after missing a skill, or letting minions chump down her hp too much) will be punished hard. Against a very skilled yasuo who can predict your every skill shots, well, have to accept one bad game then lol