Former world number nine and fellow Swiss Marc Rosset believes Roger Federer was mentally tired by the end of the ATP Finals as a result of the last one and a half years.

Federer entered the season-ending event in London as the clear favourite after world number one Rafael Nadal had to pull out with a knee problem following the Spaniard's opening round-robin loss to David Goffin.

However, Federer suffered a shock semi-final defeat at the hands of Goffin, who came back from a set down to become one of the few players to defeat both him and Nadal in the same tournament.

While it was a setback for Federer as it would have put him just 140 points behind Nadal ahead of 2018, it takes nothing away from what has been a remarkable career renaissance in 2017 after returning in January from a long injury lay-off.

The Swiss ace ended his season with a tour-leading seven titles, an Australian Open title and a record-breaking eighth Wimbledon crown — the latter two being his first Grand Slams since 2012.

Rosset believes the journey from his injury at Wimbledon last year all the way to the ATP Finals this month took its toll on the 19-time Grand Slam champion.

"I think Roger came to this Masters very mentally tired," Rosset said, as quoted on Express. "With him, we must not only talk about the past twelve months, not just talk about this season."

"To go back to the beginning of the trajectory that brought him here, we have to go back to Wimbledon 2016, the moment he gets injured.

"Everything has been chained since: the operation, the re-education, the doubt, the return to the game, then the quest for two Grand Slam titles, three Masters 1000, etc.

"How long had he not won two major in a season? At 36, all this draws a lot of energy on the emotional level. This year has been heavy for him."

Rosset adds that the last year and a half eventually led to Federer's defeat to Goffin, as the 36-year-old was simply mentally burned out.

"We also saw Roger pester like never [before] on his tennis this week. We felt that he was not happy," Rosset added. "And finally, he played Saturday against a Goffin who gave the match of his life.

"Federer is disappointed of course because he thinks it would have been nice to win the Masters, but I do not think he left London more than that. Mentally, he was simply burnt."