Madara as a last ditch effort fought Hashirama with the EMS for the first time.
He somehow was losing the fight most likely due to Hashirama's stamina as he inherited his body's perfection from the So6P AND his VAST knowledge of techniques(The Third hokage actually talks a little about this at very start of the manga
So to try and win the fight even after the element of surprise (EMS) failed he called/summoned the Fox. Now I believe at this point it was a fight between Hashirama and the nine tails as Madara surely would have been exhausted by then.
For those who don't understand how I came up the conclusion how Hashirama hasn't seen the EMS before this fight was from the order of events shown in the manga.
1) It was first shown Madara and Hashirama have fought many times.
2) Madara mentions the fights with Hashirama got him famous and the Uchiha clan's enemies grew from the encounters with the Senju clan. So Madara took he's brothers eyes to obtain EMS for the clan's protection or for himself, either case it doesn't matter.
3) The Senju clan made a truce, and the Uchiha accepted.
4) Hashirama becomes the the First hokage.
5) Madara revolts and challenges(THE FIGHT) Hashirama for the leaf village position.
He can't have had EMS before he took his brother's eyes(2).
Although they could have fought before the truce(3) and after he took his brother's eyes(2).
Also when Madara stated Hashirama gave away the bujj as peace offerings to the other villages he was shown to be wearing the Hokage gown. So this only could have happened after he became hokage(4) and either BEFORE OR AFTER the fight with Madara(5). ONLY A DETAILED FLASHBACK/GAIDEN/MOVIE CAN TELL US FOR SURE when these events actually happened.
But the most amazing thing you could say from this fight is HOW THE **** did Hashirama subdue the FOX. Now I know a lot of you are gonna say "but Hashirama could control the bujj" but Madara already was manipulating the Fox to do his biding so Hashirama would have had to attack Madara, stop him then suppress the Fox. The reason I think Hashirama would had to deal with Madara first then the fox is because of this. When the fox was under the hypnotic control from the sharingan, I was assuming Hashirama would suppress the fox's chakra like how yamato did for Naruto
in 4 tail mode. But this would not stop the mind control from the sharingan would it. From the recent chapter(490) we found out the fox has his power split up into two, mind and chakra. The sharingan would control the mind portion and therefore order the fox around, while Hashirama would only suppress the chakra part. So for Hashirama to get rid of the fox completely from the battlefield he would have had to stop the mind control from Madara by attacking Madara, then he be able to get rid of the fox from the battlefield.
So to sum up the fight, it when like this;
1) Madara (EMS) vs Hashirama ... Madara then says oh shit hes still wining.
2) Madara controls Fox vs Hashirama .... Hashirama SOMEHOW subdues the fox.
3) Hashirama knocks out Madara to stop the link between Madara and the fox.
4) Hashirama suppresses the Fox, Fox leaves.
5) Hashirama somehow with his bullshit stamina lands the final blows to an already exhausted Madara.
6) Hashirama believes Madara is dead and SOMEHOW more bullshit occurs where Madara survived the WHOLE ORDEAL.