Ben Spies said 11th in the Italian Grand Prix was the best he could do as illness overwhelmed him during the Mugello race.
The American, who has endured a difficult season on the second works Yamaha, fought for a top-six place in the early laps before tumbling down the order.
"I'm so disappointed, not just for me but for my whole team and Yamaha who have worked so hard this weekend," he said.
"I was feeling really not right from before warm-up this morning and it just got worse over the day.
"I was struggling to put three laps together at a time in the race and fighting waves of dizziness and trying not be physically sick in my helmet.
"Clinica Mobile did their best to help me before the race but it was a huge challenge to focus on the bike at the speeds we do."
Yamaha team director Massimo Meregalli was confident that Spies would have been competitive in the race if he had been in better health.
"I am very sorry for Ben and the team, they all worked hard this weekend and we were confident in a decent race pace for today," said Meregalli.
"Unfortunately Ben was pretty ill from this morning onwards and honestly put in a huge effort just to finish the race. I am confident we will be back fighting in Laguna [Seca] next week for his home race."