Boxer Sergey Kovalev agreed to a $650k settlement with the woman he allegedly beat up in 2018 -- but the woman claims he failed to come up with the cash ... and now she's suing.
TMZ Sports broke the story ... the 36-year-old Russian superstar was accused of bashing Jamie Frontz in the face during a June 2018 attack in Big Bear, CA -- breaking her nose in the process.
Frontz told police she had rejected Kovalev's romantic advances earlier in the night and he followed her back to her cabin to make her pay.