Hi. As Keiko-san said, it is hard to find a Japanese word for "deserved to win". I believe you want to cheer him up because you know how hard he practised for the match. So I will suggest what I would write in this situation based on your English sentence. 「でも、Aさん(A=your friend's name)は試合のためにすごくがんばって練習したよ。勝ちにふさわしいぐらいがんばったんだから、元気出してね。次の試合で勝てるといいね。」 You didn't write the "元気出してね。次の試合で勝てるといいね。" , which means "Cheer up! Hope you will win the next game.", but I would probably add that kind of thing to cheer my friend up. By the way, I like Nelson Mandela's word, "If you talk to a man in a language he understands, that goes to his head. If you talk to him in his language, that goes to his heart." Please cheer him up with your Japanese and it will go to his heart :)

It's hard to find the right Japanese word for 'deserve', so I change a bit as follows: あなたはこの試合(しあい)のためにとてもよく頑張っていたのを私(わたし)は知(し)っていますよ。 I know you worked so hard for this. 本当(ほんとう)に残念(ざんねん)でしたね。次(つぎ)はきっと勝(か)てますように！ I'm sorry, but I hope you'll win next time! I hope this will help! I've just started Italki yesterday and I'm not sure if it's okay to correct someone's comment like this...but here's a small correction for Martyn-San's sentence : 頑張りましたと思います。→頑張ったと思います。(keigo one time) 大変ですね。→大変でしたね。(past tense would be better in this case, but 大変ですね is grammatically okay!)

"but you worked so hard for this, you really deserved to win!" だけどあなたはこの時のためにすごくがんばっていました。あなた(の努力)は勝利に値します。

I am not Japanese, so treat my answer with extreme caution! My understanding of Japanese culture is that people don't often express the "deserved to win" mentality. They very much understand and appreciate "working hard" and "trying hard". They often use the phrase "ganbarimasu”（がんばります) and when they tried their best "ganbarimashita" (がんばりました) . When sympathising, you can say "tai hen deshita ne" (たいへんでしたね/大変でしたね）, which means "Bad luck"/"That's a shame" etc. To express sadness you could say "kanashii desu" (かなしいです/悲しいです). So, if I were saying that to a Japanese friend, I would probably say "頑張りましたと思います。大変ですね。” (ganbarimashita to omoimasu. tai hen deshita ne.) To me, it means "I know you tried hard. That's tough". I will ask one of my Japanese friends if they can help, if you don't receive any better answers.