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Video - Weir devastated with silver
Wheelchair racer Dave Weir's hopes of winning five golds came crashing down as China's Zhang Lixin pushed him into second place in their 400m final.
Weir broke his old world record but Zhang bettered it by almost a second, winning in 45.07 seconds.
Weir has been struggling with a virus all week and told BBC Sport: "I'm just not there. I'm so ill."
With the 800m, 1500m, 5000m and marathon still to come he will undergo blood tests to identify the problem.
"I've had glandular fever in the past and I know I'm susceptible to pick up anything," he added.
"I know there is more there - in training I've been quicker but it's not about training."
The defeat means Great Britain are still without a gold in the Bird's Nest.
I'd love to carry on in the sport but I can't finance it anymore
Also on day four, Stephen Payton was fifth in the T38 400m, Graeme Ballard was eighth in the T36 400m and Gemma Prescott was eighth in the discuss final.
Ballard will have another shot at gold in the 200m - his strongest event, where he won bronze in Athens.
But Payton looks set to bring an end to his career after struggling in a chaotic final, which saw three athletes fall over, including race leader Wenjun Zhou of China with 50m remaining.
"It should have been within my capabilities but it didn't happen for me," said Payton.
"I'd love to carry on in the sport but I can't finance it anymore."
In the women's T54 400m final, Canadian wheelchair racer Chantal Petitclerc suffered a scare of her own after a strong start from China's Wenjun Liu.
But the veteran came through in her to win her ninth Paralympic gold - the first of what she hopes will be six in these games.
"I knew I would have to be strong, I knew it would not be over until I crossed the finish live and that's the way it panned out," she said.