(Slight) knee raise to 1-2 or knee raise to teep then low kick.
@rmongler's observation on combos is something I didn't really understand until I started boxing in my mid 30s. Prior to boxing I learned a ton of combos in TKD and kickboxing along with some of the reasons for using them and why they work, but what I didn't learn until I was about a year into boxing is that you need to observe the opponent and figure out his reactions & patterns to really make the combos count and get the snowball effect where you land more & more until he's overwhelmed.
Nong-O is probably my favourite fighter when it comes to figuring out patterns & reactions then landing free hits and eventually taking the opponent apart.
Here he shows how to land free body kicks on someone no matter how he tries to defend. One of the sequences is a throwaway punch combo at the head to push the guy back and get him to cover his head, followed by a round kick to the open body.
Then there's one of the most vicious calculated finishing sequences I've seen in a long time. Almost every strike opens up something that he hits with the next strike, and every sequence opens up new targets for the next sequence. Chop the legs, go to the head, bang the body, back to the head, overhand rights to make him duck, uppercuts to straighten him back up, and finished with a combo to the kidney.