Russell became just the third teammate to beat Hamilton over the course of a season, and the first since Nico Rosberg in 2016, when he unexpectedly out-performed the seven-time world champion in his first year with Mercedes in 2022. 

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The young Briton has made another fast start to 2023, leading Hamilton 3-0 in the qualifying head-to-heads across the opening three races in Bahrain, Saudi Arabia and Australia. 

However, retirement in Australia has left Russell 20 points behind Hamilton in the early championship standings.

"What is true is that, in Russell, he [Hamilton] has for the first time found a teammate who can really cause him trouble,” Massa told Italian newspaper Gazzetta dello Sport.

“That only creates additional doubts as a Formula 1 driver.”

Massa, who was pipped to the 2008 world title by Hamilton at a dramatic season-finale in Brazil, has questioned whether the 38-year-old can stay motivated amid Mercedes’ continued struggles. 

Uncertainty over Hamilton’s future remains with the Briton’s current Mercedes contract set to expire at the end of the year. 

"I don't know if Hamilton will choose to stop,” Massa added. 

"It is not easy to find motivation when you have already won everything and drive a Mercedes that is struggling to reach the desired level. 

"At the beginning of his career, he was still like [Max] Verstappen and he only thought about racing 24 hours a day. However, Lewis now has many interests outside of racing."