By Jon Wertheim In this week’s mailbag, we also look at the floundering Davis Cup and ask why Nadal hasn’t gotten a solo Sports Illustrated cover. So, let’s start out by acknowledging that there’s a war going on right now. One that impacts us all. And echoes throughout tennis. A player who was in the Australian Open field to start the year, is now on the front lines. (Listen to Sergiy Stakhovsky in a few days on the SI Weekly Podcast.) Ukrainian players are competing—and competing well— despite enormous pressure and concerns. Russian players are in the brutal position of choosing between dispa…