Because Kvyat was put on hiatus by Toro Rosso because of his poor on-track performances, you'd imagine he doesn't have much leverage: but teams are contractually obliged to run two drivers and no more than four per season.
Hartley is no stranger to Toro Rosso, having tested for the team back in 2009.
Discussing his inclusion in next weekend's race, Hartley said: "What an awesome feeling!"
Hartley, meanwhile, will become the ninth New Zealander to race in F1, but the first since Mike Thackwell in 1984.
Youngster Pierre Gasly, a replacement driver this season himself, has opted to skip the race in Austin to concentrate on winning the Asian Super Formula championship at the series finale on the same weekend.
New Zealander Hartley was formerly a Red Bull junior and had previous testing experience with the team in 2009, but his recent career has nearly exclusively involved the World Endurance Championship (WEC), a title he won alongside Mark Webber in 2015. You'll note that the other driver can't be Kvyat: he has already returned in the other auto, having replaced Carlos Sainz Jr, who went off to Renault as part of a new engine deal between Honda and the two teams.
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For example, one said he felt like his brain had been "defragged" like a computer hard drive, and another said he felt "rebooted". Functional magnetic resonance imaging (fMRI) scans showed reduced activity in certain parts of the brain after taking the drug.
He went on to fashion a great record in GT racing, including his win with the Porsche factory team at Le Mans this year.
Hartley, who secured the Le Mans 24 Hours for Porsche this year, says the years he missed out on F1 made him more determined to make this opportunity a reality.
But mentor Kenny Smith reckons Hartley will be aiming for the top and will want to chase his childhood dream of racing in the Formula One.
"He is one of those who is so competitive in everything he did, he wanted to be a victor - and that is what has given him the drive to keep on going".
"He was always looking to go quicker and look what he has got now".
"He is a wonderful kid and It has been so good to associated with him over the years".
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