The victim was assaulted by a man and a woman on Glasgow's southside at around 7.30am.
He was walking outside a cash and carry on Maxwell Road, Pollokshields, with two friends when he was approached.
The man is being treated for head and hand injuries at the Victoria Infirmary where doctors described his condition as serious.
Police believe the attackers may have been involved in another altercation shortly before.
The man is described as white, aged 19-21, of medium build with short brown hair and scratches on his face. He wore grey jogging trousers and no top.
The female suspect is white, aged 20-22, of slim build with brown hair. She was wearing a light-coloured top with a dark top underneath, a skirt or shorts and black shoes.
Detective Constable Lyndsay Connor from Police Scotland said: "Our initial inquiries have revealed that the man and woman responsible may have been involved in an altercation with another man at Maxwell Road, near to its junction with Herriet Street a short time before the serious assault took place.
"The man then left in a taxi believed to be a white Skoda Octavia. This incident has not been reported to police and inquiries are being carried out to establish more details on this and to trace the victim and driver of the private hire taxi, although I would ask them to come forward to police.
"Police are continuing to study CCTV in the local area, and I would urge anyone with information that may help police inquiries, or who may recognise the description of the two suspects, to contact Police Scotland on 101 or Crimestoppers on 0800 555 111."