“I’m happy to play a singles match after so long,” Nadal said. “I need days and time to get my game back, but so far the feeling on court is great.
“For now, the most important thing is to spend as much time as possible on court. This victory allows me to play at least two more matches, singles and doubles,” he added.
The 11-time Grand Slam champion from Spain hadn’t played singles since a shock second-round exit from Wimbledon in June, although he warmed up in the tournament on Tuesday with a doubles win.
A torn tendon and inflammation in his left knee had kept him out of the London Olympics and the 2012 US Open, while a virus further delayed his return to action this year.
The rust was showing as Nadal, now ranked fifth in the world, dropped his serve in the first game of the match and quickly fell into an 0-2 hole to the left-handed Argentinian.
But he rebounded in style against his 128th-ranked Delbonis, regaining the break in the fourth game before prevailing in a hard-fought eighth game to give himself a chance to serve for the opening set.
In the quarter-finals, he’ll face either compatriot Albert Montanes or another Spaniard Daniel Gimeno-Traver.