Nico Rosberg has hailed Lewis Hamilton's "unbelievable" achievement of becoming a six-time world champion.
Hamilton moved just one behind Michael Schumacher's record haul of seven titles after a second-place finish at the US GP put him out of reach of team-mate and race winner Valtteri Bottas with two grands prix remaining in 2019.
- Hamilton wins 2019 F1 world title
- Bottas wins race but Hamilton takes title
- Will Hamilton beat Schumacher's record?
Sky F1 pundit Rosberg, the 2016 world champion and only driver to finish a season ahead of Hamilton in the hybrid era, was full of praise for his former team-mate.
"He is absolutely incredible," Rosberg told Sky Sports F1.
"We raced each other when we were 13 and 14 years old - we were best friends, we used to go on holiday together and dream big, one day in Formula 1 fighting for championships.
"But six world championships? That's just unbelievable."
Rosberg continued: "We saw all of Lewis' class out there today. He took the strategy into his own hands and maybe did a better strategy than the team was recommending him and almost got that win.
"It was a phenomenal attempt to try and get that win. He really did fantastically, lost out a little bit to Valtteri but a completely deserved sixth title in amazing fashion."
Hamilton's current contract with Mercedes runs until the end of next season, while he spoke in the week of wanting to be a "pioneer" heading into F1's new rules era from 2021.
His tally of 83 race wins is just eight behind Schumacher's all-time record of 91, and Rosberg expects Hamilton to soon be the sport's greatest driver.
"He's now closing in on becoming the greatest of all-time, and who would have thought that was even possible, for someone to even reach Michael Schumacher."
Brundle: Hamilton one of Britain's greatest sportsmen
Sky Sports F1 expert Martin Brundle said Hamilton's continued success confirmed his place as one of Britain's greatest athletes.
"Can you imagine the mental stamina and fortitude to keep pumping in world championships," Brundle said.
"So many races, layouts, conditions, challenges, you're risking your life at the same time.
"You need to be super fit. He's nowhere near the youngest man on the grid but he just keeps turning up and delivering.
"He is one of the greatest sportsmen in our country."
Don't miss the rest of the Formula 1 season on Sky Sports F1. Find out more here to subscribe