How do you pronounce "つづく"?
[tsɯ̥dzɯkɯ] or [tsɯdzɯkɯ] or [tsɯzɯkɯ]
Any comment are welcomed, as long as you know how to use IPA to transcribe human speech.
Wiktionary: [t͡sɨᵝzɨᵝkɯ̟ᵝ] I personally believe this one is possible since the pronunciation of t͡s and z is more front while k is back. The vowels are affected as a result.
ja.forvo.com/word/続く/#ja: all examples in this site gives pronunciation that omit none of the three "u" in "tsuduku".
[tsɯ̥dzɯkɯ̥] or [tsdzɯk]: This one is interesting. I also agree that pronunciative change may happen in a fast and continuous speech.
アクセント核がない、無声子音に挟まれている、という場合の /i/ /u/ が弱化・無声化する、というのは基本的な規則です。(Can I interpret it as the rule of dictionary pronunciation? I mean, when the speaker pronounce つづく isolatedly or no in a fast speecha, and hence the standard pronunciation.)