她在网球场上疾如闪电。She is fast as lightning on the tennis court.
上疾 here isn't a word; it's just 上 and 疾 happening to meet at the word boundary.
在……上 is the structure to mean "on" in this sentence.
疾如闪电 is a more literary way of saying 快得像闪电. "疾" has the archaic meaning of "fast; quick", which is rather unproductive in forming phrases and words nowadays, but you can still see it in phrases e.g. 疾如闪电, 迅疾 (a literary way of saying "fast, rapid"), 不疾不徐 (unhurried, unhurriedly; lit. not fast (and) not slow) or 眼疾手快 (agile; lit. quick eyes and quick hands).