IPA: //ˈbeɪsˌlaɪnər//
KK: /ˈbeɪsˌlaɪnər/
A person who plays tennis primarily from the baseline and rarely approaches the net.
The baseliner won the match by consistently hitting powerful shots from the back of the court.
Baseliner → The word is formed from "base" (from Latin "basis", meaning foundation or support) and "-liner" (meaning a person who). A baseliner is a person who establishes a baseline or foundation for comparison.
Think of someone who sets a 'foundation' ('base') for others to follow — that's what a baseliner does.