Great Britain captain Leon Smith admits that Andy Murray's participation in Saturday's Davis Cup doubles against Croatia is uncertain.
The world No 3 is nursing a persistent back problem which affected him during Friday's singles victory in Umag.
Murray is scheduled to partner Colin Fleming in what could be a vital doubles clash but, because of soreness in his back, he could be replaced by Jonny Marray.
The tie currently stands at 1-1, after Murray beat 16-year-old Borna Coric 6-3 6-0 6-3 in Friday's opening singles, followed by Dan Evans losing in straight sets to Ivan Dodig.
Skipper Smith said any decision regarding Murray would rest with the player himself, with Marray - the 2012 Wimbledon doubles champion alongside Fredrik Nielsen of Denmark - on standby to take over.
Smith told the BBC: "The doubles isn't until the afternoon so it gives us plenty of time to assess it.
"Andy has been feeling his back - it's an ongoing issue. He has good people around him and will get lots of treatment.
"Then it's really up to Andy. The welfare of Andy's body is far more important than this tie and we have a former Wimbledon champion who is waiting to come in should Andy not be able to play. It's not a bad situation to be in."
Marray and Fleming reached the quarter-finals of the recent US Open where they lost to the current world No 1 team - American twins Mike and Bob Bryan.