"Kept time with" is a phrase that refers to maintaining a rhythmic coordination with a person or instrument in a musical performance. Some other synonyms for this phrase could be "followed the beat with," "matched the timing of," "synchronized with," or "harmonized with." These phrases all convey the idea of staying in time and keeping pace with others in a musical setting. Using these different terms can add variety to one's writing or speaking, and help to avoid repetitive language. Ultimately, the goal is to convey the idea of staying in sync with others in a musical performance, and these phrases all do so effectively.