Murray to decide on fitness
Leon Smith will leave Andy Murray to decide whether or not he is capable of playing three games in as many days in the Davis Cup.
Captain Smith is leading GB into their first World Group quarter-final since 1986 against the United States and hopes rest largely on the two-time grand slam champion in San Diego.
Murray is still feeling his way back into the rigours of the main tour after back surgery and was last week knocked out of the Australian Open at the quarter-final stage.
Smith will need the Scot to be at or close to his best against the States, who are putting big-serving duo John Isner and Sam Querrey into action.
Murray is expected to beat both of them, but it remains to be seen whether or not he will be up to a doubles match, alongside any of Colin Fleming, Dominic Inglot, James Ward or teenager Kyle Edmund.
"That's the unknown, everyone who has spoken to Andy says he wants to play doubles, he wants to play all three matches and he's still the best player on the court," Smith told BBC Sport.
"If he's fit and healthy he plays, but we will have to see how demanding the Friday match has been and it's not until Saturday morning when he sees how his body is after the first few steps that we make a decision on it."
Amongst Smith's squad is the 19-year-old Edmund who remains an unknown quantity to many.
He is starting to make a name for himself, though, and impressed Murray at a training camp in ahead of the Australian Open.
Edmund has been selected because of his clay-court preference - the tie will be played on red clay - and Smith has no doubt he will not let himself down if called upon.
"If you asked me about his level I'd say it's there to be competitive," Smith added.
"His ranking is still low in the 300s but he's put a lot of work in for us. Whether it's him or James Ward, both of them can go out there and do a really good job for us."
Great Britain's last Davis Cup meeting with the Americans came in 1999 when Tim Henman and Greg Rusedski fell just short of a memorable win, with Rusedski losing 8-6 to Jim Courier in the deciding set of the match.
That was on the hard courts of Birmingham, with this clash at Petco Park set for a newly-laid clay court which has been causing the players some problems.
"It's not really clay, it slippy, it's quick and a lot quicker than the ones we play on in Monte Carlo, Rome and Madrid," Murray told the BBC.
"It's been different. I'm hitting the ball fine and am just getting used to the moving. It's very slippy and will be tough for everyone."
That could well prove to be true for America's two singles players, with Isner and Querrey both well above six foot.
Querrey is ready for whatever comes his way, though, including Murray, even if he has only beaten him once in six meetings.
"No matter who you play in a Davis Cup match it's tough," he told the Davis Cup website.
"At least I've beaten him (Murray) one time, so that gives me a little belief that I can do it again.
"He's got me five or six times I think but I can go out there and that will be one where I shouldn't have anything to lose. I can go out there and play aggressive and hopefully play well and I think I have a shot to beat him.
"Every rubber is equally as important, they have a great doubles line-up and Andy Murray could play... They have a lot of choices and it's not a guaranteed win for us at all, the Bryans are going to have to play really well."
The Bryans are the all-conquering Bryan brothers and, if they win, Mike Bryan will become America's most successful Davis Cup doubles player of all time with 23 points.
The States lead the overall battle 11-7 and have won the Davis Cup 32 times. Great Britain have lifted it on nine occasions, the last being with Fred Perry in 1936.