Aljaz Bedene said he struggled to cope with the shock of failing in his bid to play Davis Cup tennis for Britain as he reached round two of the Miami Open.
He beat Spain's Roberto Carballes Baena 4-6 6-2 6-3, 24 hours after being ruled ineligible to represent his adopted nation in the annual team tournament.
"I'm still shocked, to be honest," said the Slovenia-born Bedene, 26. "It's not easy, I have to tell you that."
Heather Watson plays Sloane Stephens in Miami later on Thursday.
While Bedene said he respected the International Tennis Federation ruling, he said he was angry with the way it had treated him.