Sao Paulo midfielder Oscar remained in hospital after suffering cardiac complications during a fitness test, the Brazilian club said on Wednesday.

The 34-year-old was taken to the Albert Einstein hospital on Tuesday after collapsing during an interval test on an exercise bike at Sao Paulo's Barra Funda training complex.

"Oscar remains clinically well and stable in the cardiology unit for observation, medical monitoring and investigation," Sao Paulo said in a statement.

Local media reported that the veteran playmaker lost consciousness for around two minutes before paramedics arrived at the scene. Oscar had been undergoing tests as part of his recovery from a calf injury.

His former club Chelsea was among a slew of well-wishers for the Brazilian on Wednesday.

"The thoughts of everyone at Chelsea FC are with our former player Oscar and his family at this difficult time," the Premier League club said on social media.

Oscar joined Sao Paulo on a three-year deal last December, returning to his original club after successful spells with Shanghai Port and Chelsea.

Injuries, including a lumbar fracture sustained in July, have limited the former Brazil international to 21 matches across all competitions this year.