Federer showed glimpses of the form that has seen him reclaim the world number one ranking in a 6-3 7-6(4) win over Fish, who was the last American standing at the tournament in Mason, Ohio.
Four-times Cincinnati winner Federer broke in the first and final game of the first set, looking in good shape against Fish, a hard court specialist who finished runner-up to the Swiss in the 2010 final.
Fish stuck at his task in the second set but the world number one turned it up a notch in the tie-break which he won to four.
"I'm very pleased that I was able to play at a high level and keep there throughout the match," said Federer.
Earlier, Federer's Davis Cup team-mate Wawrinka came from behind to down Milos Raonic 2-6 7-6(5) 6-4.
Reaching the last four at the Lindner Family Tennis Centre looked a remote possibility for Wawrinka after his Canadian opponent romped through the first set in clinical fashion. Raonic's game, built on a vicious serve and a booming forehand, was simple but effective.
He was bidding to become Canada's first semi-finalist at a Masters tournament or its equivalent level since 1969, and without offering up a break point during the second set, looked the favourite to close out the tie-break.
But an impressive resurgence from Wawrinka turned a 4-2 deficit into a 7-5 win, and with it the match turned on its head.
Raonic, who also had a run in Toronto the previous week, seemed to tire, and errors crept into his game. Wawrinka saw 17 aces flash by him, but when his one and only break point opportunity came up in game three of the decider, he took it, and served out from there.
"I'm playing great tennis after two tough months. I lost in the first round at Wimbledon, Gastaad and the Olympics and it wasn't easy, but I am glad to be back on the hard court," Wawrinka said.