The downturn, from which plenty suspected Rossi's career would never recover, was triggered when the Italian switched from Yamaha - where he won four of his nine world titles - to Ducati.
Prior to climbing aboard the Desmosedici Rossi's worst premier-class season was third overall, eight podiums and four wins (2007). But he took just a single podium in his debut 2011 Ducati campaign, then two rostrums the following year.
Putting his pride to one side, Rossi cut his losses and returned to Yamaha. Although he won a race in 2013 Rossi was outclassed by a team-mate, Jorge Lorenzo, for the first time in a full championship season and it took until last year for the #46 to be a consistent frontrunner.
Helped by further improvements to the M1, Rossi has stepped up another gear in 2015, winning three ra ces and finishing on the podium in all nine rounds. Rossi's score of 179 points is more than at the same stage in 2009, when he last won the MotoGP title, and his best first-half showing since 2005.
Looking back, Rossi admits it took time to recover the last 10% after the Ducati years.
"From the beginning I felt very comfortable to arrive at one level, that I think was maybe 90%," Rossi said of his Yamaha return. "But I needed time to recover the last 10%, after two years with a lot of difficulties and a different bike. That was the difficult part. To arrive at 100%."
Rossi had won 105 grands prix, including a record 79 in the premier-class, when he made the ill-fated switch to Ducati. But after two years of poor results, even The Doctor began to question if he was still good enough.
"First of all, every rider when he doesn't achieve the results that he expects, you straight away think you are not able to ride the bike at the limit any more," said the 36-year-old. "And in my case I was quite old already.
"So f**k, you think maybe now the riders are faster than you and I'm not able to reach the limit. Also with the Ducati I had a lot of problems with the front, so every time I pushed more I crashed. It was a difficult situation."
Rossi will start the final nine rounds of this season with a 13-point lead over Movistar Yamaha team-mate Jorge Lorenzo.
But the title fight could yet become a thrilling three-way contest. Honda's reigning double champion Marc Marquez is back to winning ways and, in the opinion of Rossi and many others, is more than capable of bridging even a 65-point deficit.
Valentino Rossi: Points and championship position after nine rounds in 500cc/MotoGP2015: 179 points, 1st in championship, Yamaha.2014: 141 points, 3rd in championship, Yamaha.2013: 117 points, 4th in championship, Yamaha.2012: 82 points, 6th in championship, Ducati.2011: 98 points, 4th in championship, Ducati.2010: 90 points, 5th in championship**, Yamaha.2009: 176 points, 1st in championship, Yamaha.*2008: 167 points, 2nd in championship, Yamaha.*2007: 164 points, 2nd in championship, Yamaha.2006: 118 points, 3nd in championship, Yamaha.2005: 211 points, 1st in championship, Yamaha.*2004: 164 points, 1st in championship, Yamaha.*2003: 187 points, 1st in championship, Honda.*2002: 220 points, 1st in championship, Honda.*2001: 170 points, 1st in championship, Honda.*2000: 92 points, 5th in championship, Honda.
* Went on to win world title.** Missed four races due to broken leg.
