Federer, 37, moved through the gears to win 3-6 6-1 6-2 6-2 on Centre Court.

The Swiss, aiming for a record-extending ninth men's title, will face Britain's Jay Clarke next.

Third seed Rafael Nadal faced fewer problems against Japanese qualifier Yuichi Sugita, winning 6-3 6-1 6-3 to set up a meeting with Nick Kyrgios.

Federer was 'frozen' as Harris threatened seismic shock

Harris, 22, had never even won a match on grass going into his meeting with arguably the greatest player of all time.

But the world number 86 rocked Federer with blistering serves consistently around 130mph and backed that up with some heaving groundstrokes in a first set which left Centre Court murmuring in disbelief.

After breaking Federer's serve for a 4-2 lead, Harris confidently sealed the opener in 29 minutes as he threatened to create one of the most seismic shocks ever seen at Wimbledon.

Federer, who had dropped a set in the first round at Wimbledon for the first time since 2010, seemed to lack the assurance which has rarely gone missing in his illustrious career.

"I struggled early on. I felt a bit frozen, my legs weren't going," he told BBC Sport.

"I felt it was heavy out there, the ball wasn't going when I was hitting it and he was hitting it big."

The challenge for Harris was always going to be maintaining his level as Federer started to find more rhythm and make him work harder for his points.

As Federer rediscovered his timing and touch, Harris wildly slashed a forehand wide - with the ball sitting up invitingly for the South African and Federer stricken at the baseline - on a break point for a 3-1 lead to the Swiss.

From that point he could not recover, Federer winning 11 of the next 13 points to level the match and then continue to go through the gears as he broke twice in the third set.

Then it turned into a procession towards victory, with Harris looking like he might have to pull out with a calf injury which left him needing lengthy treatment.

He refused to quit as Federer began to purr, a dipping, cross-court backhand winner sealing another break early in the fourth for the Swiss as he cruised to his 17th straight first-round win at the All England Club.

"He kept me nervous for a set and a half and I think that break was crucial in the second to turn things around," Federer said.

"It took a big effort. Lloyd played a good match."

Nadal's bid for a third title up and running

Nadal also made a slow start as 30-year-old Sugita broke his opening serve in the first game, with the world number 274 threatening to go 3-0 up before the Spaniard fought back from 0-40 to hold.

The sense of shock around Court One did not last much longer though, 33-year-old Nadal breaking back in the next game and taking control with four of the next five to clinch the opening set.

Sugita, who had only won one previous match at SW19, offered little resistance from that point as two-time champion Nadal raced through the second set and dominated the third, crucially breaking in the eighth game.

That set up a tantalising meeting with controversial Australian Kyrgios, who beat compatriot Jordan Thompson in an entertaining five-setter earlier on Tuesday.

Nadal accused Kyrgios of "lacking respect" after the 24-year-old beat him in Acapulco earlier this year, with Kyrgios describing the 18-time Grand Slam champion as "super salty" in a recent podcast.

Asked about the attention surrounding their relationship, Nadal said: "Being honest, I'm too old for all this stuff. What I said, I said. That's all.

"I'm not a guy who will be in a fight with anybody. I will keep doing things I love. On Thursday I will be back in this amazing place.

"At the beginning of this match it was tough. There are lot of things to improve."