Novak Djokovic found himself back in the mire but mounted a Davis Cup fightback before Russia's Daniil Medvedev retired due to injury, while Italy left defending champions Argentina facing a first-round exit on Friday.

Playing his first match since a sensational second-round loss to Denis Istomin at the Australian Open, Djokovic came from a set and a break down to lead 3-6 6-4 6-1 1-0 when 20-year-old Medvedev was unable to continue due to a leg injury.

The 12-time grand slam champion had treatment on a shoulder problem early in the second rubber and was in danger of enduring another embarrassing loss when he trailed 3-0 in the second set.

Davis Cup debutant Medvedev, ranked 61 places below Djokovic, silenced a partisan crowd in Nis before the world number two swung the match in his favour.

Serbia are well-placed to reach the World Group quarter-finals after Viktor Troicki overcame Karen Khachanov in a five-set thriller to put them in front at Sportski Centar Cair.

Argentina beat Croatia to end their wait for a first Davis Cup title last November but will need a remarkable fightback in order to avoid being brought back down to earth in the absence of Juan Martin del Potro.

Paolo Lorenzi and Andreas Seppi put Italy in command in Buenos Aires with victories over Guido Pella and Carlos Berlocq respectively.

Even the presence of Diego Maradona among a crowd of 2,000 was unable to inspire the hosts, with Lorenzi winning 6-3 6-3 6-3 before Seppi's 6-1 6-2 1-6 7-6 (8-6) success.

It is honours even in Osijek, where Roberto Bautista Agut brought Spain level with a straight-sets win over Ante Pavic after Franko Skugor put Croatia in front by beating Pablo Carreno Busta in a five-setter.

Australia lead fourth seeds the Czech Republic 2-0 following straight-sets wins for Jordan Thompson and Nick Kyrgios, while France also won both rubbers against Japan. Belgium and Germany are locked at 1-1.

In Birmingham, the United States are up 2-0 over Switzerland after John Isner defeated Henri Laaksonen 4-6 6-2 6-2 7-6 (7-1), while Canada and Great Britain are locked at 1-1.