The Paralympic double gold medallist has had a hectic schedule at London 2012 but managed to win his heat to secure a place in the final at the Olympic Stadium.
"I had to treat it as a final really," Weir told Channel 4. "I had to keep remembering it was two spots [going through to the final], not three.
"I'm a bit tired today but I've got through. I got a bit more sleep than the first night so I feel a little bit more recovered. I just need a little bit more food tonight and I hope I'll be all right for the final."
Weir won his first gold at London 2012 in the men's T54 5,000m final.