That wasn't a straight fight as Lewis started six places behind him on the grid. One of the requirements of a straight fight, in my view, would be for them to both start on the front row. You could argue that, looking at the whole race weekend including qualifying, Rosberg earned his place further up the grid and thus "beat" Lewis fairly.
Well I'm just looking at the races themselves and I still don't believe Rosberg has beaten Lewis in a straight fight this year.
Lewis retired in both Australia and Canada through no fault of his own (don't try to argue about Canada - it was purely a timing issue with his pitstop allowing his brakes to cook rather than "lesser" management skills).
Monaco was highly dubious because of the qualifying question marks but, even so, Lewis was clearly faster than Rosberg for the first part of the race and it was only the nature of the circuit which allowed Rosberg to stay ahead - anywhere else Lewis would have had him fairly easily.
So no, Rosberg has yet to beat Lewis in a straight fight where they both start in adjacent grid positions and have cars which both perform the same. All the talk by Brundle/Crofty on Sky about Rosberg "proving he has the tools to beat Lewis" was utter twaddle - he's proven no such thing so far, he's just been very lucky.