Williams technical chief Paddy Lowe believes Lewis Hamilton has "raised his game" since losing the 2016 title to team-mate Nico Rosberg.
Lowe worked with the five-time world champion at McLaren and Mercedes until 2016, believes that Rosberg's win a couple of years ago "strengthened his focus on his approach to racing".
Since the defeat to Rosberg, Hamilton has won both titles consecutively and Lowe believes he's become harder to beat.
"I think he's driven two terrific seasons," Lowe told RaceFans. "Two of his strongest seasons of all these last two years. The 2016 year was one he didn't win and he probably believes he could have won it so that was a good motivation for him to understand how to strengthen his game. And we've seen that over the last two years."
Sebastian Vettel was Hamilton's closest competitor this season, and Lowe believes that the Brit has "outclassed" his championship rival over the past two seasons.
"He's become a very difficult driver to beat," Lowe continued. "We've seen Sebastian has struggled to beat him this year. There have been losses in all sorts of different areas, but even just as a driver matching I think Lewis has outclassed Sebastian.
"That's been one of the mismatches, there have been other mismatches in terms of the teamwork. One team has performed better than the other team top to bottom. But on top of it all is Lewis, who's just a phenomenal driver."