Great Britain are on the verge of reaching the Davis Cup World Group quarter-finals after a five-set win by James Ward over Sam Querrey of the USA.
Andy Murray beat Donald Young 6-1 6-2 6-3 in the opening rubber of the best-of-five tie on the San Diego clay.
Ward then fought back from two sets to one and a break down to beat Querrey 1-6 7-6 (7-3) 3-6 6-4 6-1.
Britain lead 2-0 and need one point from Saturday's doubles or Sunday's two reverse singles to win the tie.
Murray and Colin Fleming are scheduled to face world number one pairing Bob and Mike Bryan in the doubles, although captain Leon Smith could rest Murray, knowing a singles win for the Wimbledon champion over Querrey will be enough to secure the tie.
"That's a conversation that I'll have with the guys tonight and tomorrow morning," Smith told BBC Sport. "I'm pretty open about it."
To have such a choice is an unexpected luxury for the British team, and comes as a result of world number 175 Ward's stunning win over the 45th-ranked Querrey.
Despite battling through a tie-break to win the second set, Ward looked to be out of it when he trailed the big-serving American by a break in the fourth set, and he almost went a double-break down at 0-40.
But an inspired run of eight games in a row turned the match in the Londoner's favour, and he dominated the final set before firing away a smash on match point.
Asked if he had always believed he could fight back, Ward said: "I did. It's easy to say but I played him in the past and beat him in three sets.
"At one point he was serving unbelievably and I couldn't really get near him. I held serve from 0-40 and that was key. I started reading the serve a little bit better. It's just a great win. I dug in and gave myself a chance. In the end it worked."
He added: "I love playing for my country and it shows in my performance."
Smith had surprised many by picking Ward, 26, over the 19-year-old Kyle Edmund, who is ranked far lower but has been enjoying success playing on clay.
It was Ward's Davis Cup record of eight wins in 13 singles matches, including a five-set win over Russia's Dmitry Tursunov last year, as well as good form in practice that got the Londoner a starting place.
"That's why he got picked at the end of the day," said Smith. "I don't want to take any credit, I just sit there. It's down to James.
"He kept believing in himself and we just talked about being the aggressor, being the boss. He won a lot of the points when he got Querrey on the back foot. He's been brave, he's been courageous."
In the opening rubber, Murray had far too much for Davis Cup debutant Young, the world number 79 who replaced injured world number 13 John Isner.
"It's important to get off to a good start," said the Scot.
"The court is still pretty slippery but it's been raining, it's colder, there's a lot of cloud cover, so the court's obviously going to play a bit slower. The conditions being like this helped.
"We'll have to see about the doubles. I'll speak to the captain and the rest of the team and see what the best way forward is."
Saturday from 20:00 GMT
Bob Bryan/Mike Bryan v Andy Murray/Colin Fleming
Sunday from 19:00 GMT
Sam Querrey v Andy Murray
Donald Young v James Ward