The Italian has finished on the podium just twice since moving to Ducati, and after crashing out at Laguna Seca - having been running a disappointing eighth – he admitted staying motivated was tough.
Having made the decision to return to Yamaha, with whom he enjoyed unprecedented success between 2004 and 2010, Rossi says an inability to improve upon his current level of performance would serve as an indication that his time at the top has passed.
"If you ask a sportsman whether going badly for a long time has an influence, he will tell that no, it has no effect. Instead I say it has, you can feel it," Rossi told Gazzetta dello Sport.
"This is why before talking about victories and a tenth title, which would be a dream, I need to really understand whether I'm able to fight again for the top grid positions, for victories, and finally for the title.
"Have I considered I won't be able to? Honestly I haven't made any programmes. I want to have fun again and go to the races with a smile.
"Should I carry on scoring fifth and sixth positions, that would mean my time has passed and I will retire: I've been racing for 15 years after all."
Rossi apologised for his inability to bring success to Ducati, but said he hoped he could rise again from such a low with Yamaha.
"I'm sorry this Italian pairing hasn't worked for myself, for Ducati, for the fans, and most of all for the guys who have worked on this project," he said.
"These two seasons are very difficult, and we struggling very much. Unfortunately, we were not able to improve our speed, our performance, and to fight for the good position, for the front position.
"In Ducati I find a lot of good people. We had great times together. We try the maximum, but unfortunately we were not able to achieve the result.
"I have never been able to make the difference like in 2004 at Yamaha, but the situation has changed. The bikes count for more now than back then. Also, in 2004 I was 24 and at the top of my form. Now I'm 33...
"I'm a rider who has lived twice: I won five consecutive titles like only two others have managed in history. Then I lost, but I returned and won again. Let's hope I become the man who lived three times."
Rossi said he expected the majority of his crew to follow him across to Yamaha, as they did when he moved from the Japanese factory to Ducati.
"So I think my crew come with me - more or less the same guys that come with me from Yamaha to Ducati. But is still not decided 100 percent. I think we have to fix some of the ties," he told a press conference ahead of the Indianapolis GP.
He also paid tribute to Jorge Lorenzo, admitting he is unsure whether he will be able to best the Spaniard once at Yamaha.
"Lorenzo has been a gentleman, he hasn't thrown a spanner in the works. Would I have done the same with roles reversed?
"In the last seasons, Jorge has become more strong than when I was with him in Yamaha – [and] in 2010 he was very strong. He rides the M1 at an incredible level without doing any mistakes. So for me it's a question mark [beating Lorenzo], I don't know.
"In the last seasons I think mainly three riders make the difference - him, Dani and Stoner. I was with them until 2010, so I consider [that] I was a top rider. But after these two seasons, no, I don't know."
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