Weather kept court covers firmly in place at the Foro Italico as impatient fans milled around the grounds awaiting the Swiss player's second-round match against Portugal's Joao Sousa.
Federer, who has played four Rome finals but never won a title, last played the event in 2016, losing a third-round match to Dominic Thiem.
He added Rome as a late addition to his schedule to get more accustomed to the style of play which will be required at the French Open, which begins a week from Sunday.
Last week in Madrid Federer reached the quarter-finals, losing to Thiem. Federer came back to clay in the Spanish capital after skipping two seasons to better prepare for the summer grass tournaments in 2017 and 2018.
Second seed Rafael Nadal was also due to make a Wednesday start, facing Frenchman Jeremy Chardy. Top seed Novak Djokovic was to kick off against Canadian young gun Denis Shapovalov.
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