Fellow scrum-half Greig Laidlaw was Scotland's man of the match in their opening Pool B win over Japan.
But Pyrgos takes over as skipper as head coach Vern Cotter rotates his squad against the USA on Sunday.
"Greig had a great game on Wednesday, but I'll focus on my role and do it to the best of my ability," said the Glasgow Warriors 26-year-old.
"I just look at it like it's a great opportunity.
"Hopefully, we get the right result."
Pyrgos led Scotland in a World Cup warm-up defeat by Ireland last month and will be determined to retain his place.
However, Laidlaw is almost certain to return for the potentially decisive meeting with South Africa in Newcastle a week on Saturday.
"I knew coming in that was how it was going to go," added Pyrgos.
"It was amazing to lead the team out against Ireland and again it's a huge honour to captain Scotland at a World Cup.
"I know how much it means to the country and I'm really excited.
"I'm not sure how I'll feel when I get out there, but it'll be a big moment for myself and my family."

Farmers, conservationists and the RSPB have worked together to preserve the breeding habitat in fields that has helped the stone curlew survive.
The birds, which winter in Africa, return to Breckland farms in April.
Fields have been specially ploughed for birds to get at worms and insects while plants that attract them line edges.
Those involved in the conservation programme met in Thetford on Friday to celebrate and welcome the arrival back of the birds.
Breeding pair numbers have almost trebled and the RSPB has described the project as pioneering.
Source: RSPB

Her Royal Highness took the baton at the Hamburg Symphony Orchestra's new home, the Elbphilharmonie concert hall.
It was part of an event where 250 schoolchildren were introduced to music in front of the Duke and Duchess.
The Royal party are due back in the UK on Friday evening, after a five-day tour that began in Poland on Monday.
On the last day of the tour, the Duchess, who comes from a musical family and played the flute at school, took the musicians through the first notes of Beethoven's fifth symphony.
The royal couple then listened to a performance of a symphony especially adapted for young people, and then went on stage to learn more about the orchestra.
Earlier, Catherine and William walked around the stage accompanying six children trying out different instruments, including a violin, flute, timpani [kettle drums] and trombone.
Later the Duke of Cambridge gave Prince George and Princess Charlotte a guided tour of a helicopter at the Airbus factory in Hamburg.
Prince George tried on a pilot's helmet while Princess Charlotte played with buttons in the cockpit.
The Duke flew helicopters when he was an air ambulance pilot.
Antoine van Gent, head of flight testing at Airbus, said: "The Duke was very relaxed showing his children the professional aircraft he uses.
"George was excited, with the first helicopter he wanted to sit in the cockpit and then he wanted to sit in the next one, he already knew there was a difference between them."
Charlotte was less impressed after taking a tumble on the concrete after leaving the aircraft but Mr van Gent said she enjoyed playing in the pilot's seat.
Henry Pyrgos is ready to make the most of what could be his only chance to captain Scotland at this World Cup.
[NEXT_CONCEPT]
A bird that was on the road to extinction in its native habitat in the Brecklands of Norfolk and Suffolk is thriving after 30 years of care.
[NEXT_CONCEPT]
The Duchess of Cambridge tried her hand at conducting a prestigious symphony orchestra on the final day of the royal tour of Germany.