This study aimed to examine (a) the developing interrelations between the efficiency of reading individually presented words (i.e., isolated word recognition speed) and the efficiency of reading multiword sequences (i.e., word list and text reading fluency); (b) whether serial digit naming, indexing the ability to process multi-item sequences, accounts for variance in word list and text reading fluency beyond isolated word recognition speed; and (c) if these patterns of relations/effects differ between two alphabetic languages varying in orthographic consistency (English and Greek). In total, 710 Greek- and English-speaking children from Grades 1, 3, and 5 completed a serial digit naming task and a set of reading tasks, including unconnected words presented individually, unconnected words presented in lists, and sentences forming a meaningful passage. Our results showed that the relation between isolated word recognition speed and both word list and text reading fluency gradually decreased across grades, irrespective of contextual processing requirements. Moreover, serial digit naming uniquely predicted both word-list and text reading fluency in Grades 3 and 5, beyond isolated word recognition speed. The same pattern of results was observed across languages. These findings challenge the notion that individual word recognition and reading fluency differ only in text-level processing requirements. Instead, an additional component of processing multi-item sequences appears to emerge by Grade 3, after a basic level of both accuracy and speed in word recognition has been achieved, offering a potential mechanism underlying the transition from dealing with words one at a time to efficient processing of word sequences.