We were in the lead, but we made a huge mistake towards the end of the game, and the tables were turned.
We were dominating the game, but then we made a big mistake as the game was ending, and the other team came in from behind and won.
We were on course to win, but we messed up as the game was coming to a close and the other team turned the tables on us.
英訳① in the leadは「首位に立って」、make a huge mistakeは「大きなミスを犯す」、towards the end of the gameは「試合の後半で」という意味です。
The tables were turned.はもちろん文字通りの意味ではなく、「形勢が逆転した」ということ。
英訳② dominate a gameは「試合を優位に進める」の意味。
as the game was endingは「ゲームが終わるにつれて」、つまり「終盤で」ということ。
come in from behindは「後ろからやって来る」、つまり「追い上げてきた」という意味です。
英訳③ on course to ...は「順調に…しているところで」、mess upは「しくじる」という意味です。
turn the table on ...は①の変化形で、「（相手チームなどが）…を逆転した」となります。
その他の言い方：
- We were in the lead, but we made a big mistake as the game was ending and there was a huge upset.
*upset「番狂わせ」
アメリカ合衆国 
