The 175th-ranked Ward sealed his stunning victory over the 49th-ranked Querrey in just over three hours after Wimbledon champion Andy Murray had crushed Donald Young, the replacement for the injured John Isner, 6-1 6-2 6-3 in the opening singles match at Petco Park.
Ward let out a deafening scream after ending the match on the red clay surface with an overhead forehand smash, having improved his Davis Cup singles record to 9-5.
Asked in a courtside interview how he had managed to claw his way back from two sets to one down and trailing 2-4 in the fourth, Ward replied: "I'm not quite sure at the moment.
"In the third (set), I started reading it a little bit better, he was going to my backhand quite a lot on every first serve. So I started to stand back a little bit further to give myself a little more time."
Ward was delighted to have been brought in by Britain's Davis Cup captain Leon Smith as the number two player behind the sixth-ranked Murray.
"I think he (Smith) has got confidence in me," said the 26-year-old, who had upset Querrey 3-6 6-3 6-4 on the grass courts of Queens' Club in 2011 in their only previous meeting.
"I have played these matches before and I've won a few big matches in Russia to get us here. It's always a proud moment (playing Davis Cup), so when he decided to pick me, it was great news."
Murray had set the expected winning tone against Young in the first match, powering down seven aces and hitting 33 winners to seal victory in one hour 38 minutes against his 79th-ranked opponent.
Though Young came up with 25 winners, 18 of them on his forehand, he piled up 55 unforced errors as Murray improved his Davis Cup record to 17-1 in singles and 20-6 overall.
Querrey then breezed through his opening set against Ward but lost the second on a tiebreak, which was won 7-3 by the Briton after the American blasted a forehand long.
Roared on by the US fans, Querrey stepped up a gear to win the third set and he went up an early break in the fourth before losing serve in the eighth and 10th games as Ward levelled the match.
A fired-up Ward again broke Querrey in the second and fourth games of the fifth set to lead 4-0 before Querrey broke his opponent's serve on a fortunate net cord to end a run of eight games lost.
However, the American again failed to hold in the sixth game and Ward served out to put his team 2-0 ahead.
In Saturday's doubles, Americans Bob and Mike Bryan are scheduled to meet Murray and Colin Fleming before the reverse singles are played on Sunday when Querrey comes up against Murray and Young faces Ward.
The United States have won the Davis Cup a record 32 times and Britain will be bidding for their first victory over the Americans in the competition since 1935.
- Sports & Recreation
- Andy Murray
- Donald Young