しあいは勝ちそうでした。- meaning: I was about to win the game.
How would I say:
I was not about to win the game, (but at the last minute I won).
A) しあいは勝ちくなさそうでしたけど...
b) しあいは勝ちそうじゃなかったですけど...
C) しあいは勝たなさそうでしたけど...
As an English speaker, how am I to guess what each of the above mean? Is it obvious if I just think hard enough?
Although [勝]{か}ちそう is grammatical, the native speakers' word choice would surely be 勝てそう in this particular context.
A) しあいは勝ちくなさそうでしたけど... makes little to no sense. One cannot say 勝ちくない. With a く in there, it looks as if you conjugated a verb as you would an adjective. If you said this out loud instead of writing it, we might not understand. The 勝ちく part would just throw your listener.
B) しあいは勝ちそうじゃないでしたけど... makes more sense than (A). This is ungrammatical as Snailboat stated in the comment above. We would, however, understand it if you said it. The grammatical phrase is 勝ちそうじゃなかったけど.
C) しあいは勝たなさそうでしたけど... is 100% grammatical. Whether or not this sounds natural, however, is another thing because of what I said at the beginning.
In conclusion, even though (C) is best as far as grammar without correction, (B), if corrected, takes the cake for the naturalness of the structure choice. By using 勝てる instead of 勝つ to form a 勝てそうじゃなかった, it would be best.
From "A Dictionary of Basic Japanese Grammar"
The negative nai 'not exist/not' changes to nasa before そうだ:
村山さんの家はあまり新しくなさそうだ。
Mr. Murayama's house doesn't look so new.
In this construction, the negative forms of verbs usually don't precede そうだ. Instead, Vmasu sou ni/mo nai is used:
クリスは車を売りそうにない。
クリスは車を売りそうもない。
Chris doesn't seem to sell his car.
この問題は学生には出来そうにない。
この問題は学生には出来そうもない。
It doesn't seem that the students can solve this problem.
I'd consider
勝てそうにない / 勝てそうになかった
which is obviously related to 勝てそうじゃない（なかった） but has the additional subjective nuance of "no chance of winning".