How does a zygote become diploid?

I understand the process of meiosis, where a diploid cell divides into two haploid cells, each haploid cell with two sister chromatids. In meiosis II these sister chromatids split into two single chromatids, or single chromosomes to produce four gametes. My question involves how the zygote is diploid. If two gametes fuse (two single chromatids/chromosomes), wouldn't this be what is seen in the end of meiosis I/beginning of meiosis II, where there are two sister chromatids fused as a haploid cell? The book says a zygote is diploid, so I am rather confused here. An elaborate answer would definitely be appreciated! If I'm not explaining myself well, please let me know and I'll clarify.