Which shows predate Sword Art Online that have the same basic plot? The most similar anime show I could ever find (protagonist trapped in MMORPG world) is .hack//Sign Protagonist wakes up with slight amnesia in MMORPG world and tries to find answers as to what happened to him and why he can't log out of the game. It was released on April 4 2002 (an early version of the first volume of SAO was supposedly made around 2001, .hack//Sign started development in as early as 2000).
There are plenty of other isekai manga, anime and light novel earlier than SAO but I can't seem to find another one that is as close other than .hack//Sign as of now.
Has Reki Kawahara ever said that he drew inspiration from a particular show? It was not directly stated by him but according from a biography of him that I found, it was mentioned that Kawahara liked playing games.
Kawahara would often head to the arcade with his fellow members to play fighting games. He also started playing online games in 1998, starting with Ultima Online. Kawahara also played Ragnarok Online, his most played game (although he stated World of Warcraft during Sakura-Con 2013), and Phantasy Star Online.
From this, we can say that his time playing these games could possibly have an influence to SAO, which is in a similar setting. It was not mentioned that he was influenced by anime shows when he wrote SAO. It was, however, mentioned, on the same biography, that he was inspired by some works when he wrote Accel World.
In 2008, Kawahara decided to write again for the Dengeki Light Novel Award with Kyouzetsu Kasoku Burst Linker. He reapplied because he was inspired by all the works he read from Media Works Inc., such as Tokyo Shadow and Those who Hunt Elves.