Krajinovic reacted well to Federer's first blow, breaking back immediately after going 2-0 down at the beginning of the match.
Federer returned to stadium court on Sunday afternoon and needed an hour to finish off his second round victory 6-3, 7-6 (8-6).
"I feel like when you do come back from injury or when you haven't played in a long time, it just takes extra effort".
Dimitrov, victor of the ATP World Tour Finals in 2017, lost the first set on a tie-break 7/4 but took the second 6-4 after breaking the 34-year-old on both of his first two service games.
In spite of the loss, the former world number one was able to take away some positives.
Open slump. Federer improved to 65-5 since returning from a left knee injury a year ago with his third-round victory in a rare day match. His win over Djokovic was his first main draw win in a tournament since an ATP Challenger event in October.
He extended his 2018 win streak to 13 matches and has only dropped three sets from 34 played this year. He lost to No. 109 Taro Daniel of Japan 7-6 (3), 4-6, 6-1. "I care about the moment".
"In Rotterdam I played five matches in a row", he said.
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"I think it was a good match".
His third-round opponent is Argentina's Leonardo Mayer, who had been scheduled to play Kei Nishikori in the second round before the world No. 25 withdrew through illness.
Federer made short work of Filip Krajinovic to reach the fourth round on Monday, winning 6-2, 6-1 in just 58 minutes.
He then took Federer to deuce in the next game.
There aren't many players who can say they have a winning record over 20 time Grand Slam victor Federer but Delbonis is one of them. "Very weird", Djokovic said.
Marin Cilic is also through after a straight sets win over Marton Fucsovics 7-5 6-3 and Milos Raonic beat fellow Canadian Felix Auger-Aliassime 6-4 6-4.
Yuki Bhambri of India pulled off a surprise when he toppled ninth-seeded French ninth seed Lucas Pouille 6-4 6-4.
Likewise, 23rd-seeded Hyeon Chung of South Korea dispatched 12th-seeded Tomas Berdych of Czech Republic 6-4, 6-4, despite 12 aces against just one double fault from Berdych.