The defending champions held off a late fightback from Worcester Warriors on Sunday to secure a bonus-point victory with a 32-15 scoreline at Welford Road.
Thompstone, who scored his side's first try of the afternoon, admits that the Tigers must improve on their performance.
"I don't think we played our best, that's for sure, but I knew the bonus point was coming," Thompstone told reporters.
"I think we did take our foot off the pedal in the second half, it was a bit flat out there and we can't really afford to do that this season."
Leicester face Bath at the Rec in their next match on Saturday.