The second seed broke six times to rally against Harris, the 22-year-old who was an alternate at last year's Next Gen ATP Finals in Milan and was making his Wimbledon debut.
The 6'4” right-hander, however, had the Centre Court crowd murmuring as he returned aggressively and broke in the sixth game, the first time Federer had been broken in the first round of Wimbledon since 2012 against Spain's Albert Ramos-Vinolas.
Harris then took the opener, marking the first time Federer had lost his first set at Wimbledon since 2010 against Colombian Alejandro Falla, a match the Swiss won in five sets.
But Federer settled down quickly and went on attack against Harris, whose level slipped after the opener. Federer won 29 of his 31 net points (94%) for the match and finished with 41 winners to only 14 unforced errors.
“I just felt like [the court] was slow. I couldn't really have any impact. He was doing a good job of returning me... I wasn't hitting my spots, and he was reading my serve, or he was just doing a good job,” Federer said. “I just struggled. As my legs weren't moving, it's hard to see the short ball. In defence, you're weak. The next thing you know you're struggling. That's what I had going.
“But I think with my experience I stayed calm. I know I have other things in the bag that I can come up with, other tricks. I just took a bit of time.”
The Swiss won his 10th NOVENTI OPEN title in Halle on 23 June for his 102nd tour-level title and 19thon grass. Five of the eight times Federer has won Wimbledon, he's won Halle as well. Federer will next meet #NextGenATP Brit Jay Clarke, who beat American Noah Rubin 4-6, 7-5, 6-4, 6-4.
It will be their first FedEx ATP Head2Head meeting. Federer won the Wimbledon junior boys' title the in 1998, the year the 20-year-old Clarke was born.
“I wasn't thinking about it at all, to be honest,” Clarke said of the potential Federer matchup. “I know how tough Noah is to play. I've seen him play a lot of the same ATP Challengers as me. Like I said, to beat him would have been a good win. I'd have been happy with that. He was a bit of extra motivation to get through.”
In other action, Germany's Dominik Koepfer beat Serbian Filip Krajinovic 6-3, 4-6, 7-6(9), 6-1 and will next meet 24th seed Diego Schwartzman, who beat Aussie Matthew Ebden 6-4, 3-6, 6-3, 6-2.
Did You Know?
Federer has not lost in the first round of a Grand Slam since 2003 Roland Garros (l. to Luis Horna). The Swiss has lost in the Wimbledon first round three times: on debut as a wild card in 1999 (l. Jiri Novak), in 2000 (l. Yevgeny Kafelnikov) and as the seventh seed in 2002 (l. Mario Ancic).