Do you think he was actually fighting minato at his best/seriously Or his motive was just to summon the fox on the village? and bear in mind he has the MS at this point.
I highly doubt he was fighting minato seriously. Obito's Genjutsu is strong enough to actually kill the opponent (vs Konan)
Being that Minato has no knowledge of Uchiha's secret technique's A simple 'Izanagi + Tsukyomi that could kill' combination should catch him off guard.