Pete Waterfield's illness coming into the 10m synchro final at the Fina World Championships appeared to tell as he and partner Tom Daley finished sixth.
Waterfield, 30, had been suffering from flu-like symptoms for days but decided to compete at the last minute.
"It was not their day, they were never going to be great with such a disruptive preparation," said BBC Sport's Leon Taylor in Shanghai.
China won the gold medal on home soil with Germany second.
In the developing British duos' competitive debut in January they won the National Cup in the 10m synchro, and then in April - in only their second international competition together - they claimed a dramatic gold medal in the Diving World Series event in Sheffield.
Daley then missed the 10m synchro event at the National Championships in June following the death of his father, Rob.
Speaking to the BBC earlier in July, Waterfield said he and Daley were "still learning about each other" and would go to Shanghai to "put in a good performance and learn about the event".
That was evident, although more telling was the valuable training time they were denied because of Waterfield's illness.
"They should be challenging for medals in a year's time at London 2012," added Taylor, Waterfield's former partner when they won a silver medal at the 2004 Athens Olympics, on the BBC Red Button.
The synchronisation of Daley and Waterfield, who are due to compete against each other in the 10m individual event, was out of tune on occasion at the outdoor diving pool at the Oriental Sports Centre.