Britain's Andy Murray says it will "be tough" to play in Saturday's Davis Cup doubles after losing a marathon five-hour singles match against Argentina.
The world number two was beaten in five sets by Juan Martin Del Potro, while Kyle Edmund also lost as Argentina went 2-0 up after day one in Glasgow.
Murray and his brother Jamie are set to play doubles at 14:00 BST, but line-ups can change an hour before the start.
"I'll have to see how I feel when I wake up in the morning," said Murray.
"I've never played a match that long. I've played matches close to that length but none after an extremely long stretch of playing, so I don't know how I'll feel."
Guido Pella beat Edmund 6-7 (5-7) 6-4 6-3 6-2 to ensure Argentina made the perfect start on Friday against the defending champions.
Britain must win Saturday's doubles and both singles matches on Sunday if they are to keep alive their hopes of back-to-back Davis Cup triumphs.
Captain Leon Smith said Dan Evans was ready to step in to the doubles if Andy Murray, who has the reverse singles against Pella on Sunday, is rested.
Smith told BBC Sport: "Dan's done the most amount of additional doubles practice this week. That was the thought process, so there's not too much surprise there but we'll wait for the morning."
Murray's match against Del Potro was the longest of his career at five hours and seven minutes, and his second five-set defeat in nine days following his loss to Kei Nishikori at the US Open.
It was also Murray's first Davis Cup defeat on home soil and broke his 14-match winning streak in the competition.
"I'm very proud of how I fought, I did fantastic," said the Scot after his 6-4 5-7 6-7 (5-7) 6-3 6-4 defeat.
"I fought for every point, tried as best as I could. That's all you can do. It was very fine margins. That happens in tennis and sport sometimes. It could have gone either way and he just played a little bit better in the fifth set."
|Great Britain 0-2 Argentina|
|Friday: Juan Martin del Potro beat Andy Murray 6-4 5-7 6-7 (5-7) 6-3 6-4|
|Friday: Guido Pella beat Kyle Edmund 6-7 (5-7) 6-4 6-3 6-2|
|Saturday: Andy Murray & Jamie Murray v Federico Delbonis & Leonardo Mayer|
|Sunday: Andy Murray v Guido Pella|
|Sunday: Kyle Edmund v Juan Martin del Potro|