World number two and top seed Federer defeated Germany's Florian Mayer 6-0, 7-6 (7/1) as he continues his build-up to an assault on an eighth Wimbledon title.
The Swiss required just 19 minutes to take the first set against Mayer before going on to conclude the tie in a little over an hour.
Karlovic battered his way into the semifinals by smashing 45 aces past a bewildered Tomas Berdych en route to a 7-5, 6-7 (8/10), 6-3 victory.
The 36-year-old Karlovic, who stands 2.11m (6ft 11in) tall, won the match with the last of his aces and now has 94 on the Halle grass courts from three rounds of tennis.
“I'm extremely happy about the record,” Karlovic said of his battery of aces, a new record in a best-of-three set match.
“Hitting aces helps a lot, you can avoid the volleys. I was surprised when I was told on court after the match how many aces I hit. I didn't know it was that many. Hopefully I can continue like that in the semifinal.”
His record came just two days after Germany's Sabine Lisicki blasted 27 aces in a match in Birmingham — a new record for the women's tour.
Japan's world number five Kei Nishikori also landed in the semifinals after beating Poland's Jerzy Janowicz 6-4, 5-7, 6-3.
Second seed Nishikori will next take on Andreas Seppi after the Italian's opponent Gael Monfils, the 16th-seeded Frenchman, retired from their quarter-final with a leg injury suffered when he slid into a courtside
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World number 51 Seppi was leading 6-1, 1-0 at the time.
Last year, after beating Monfils in the quarters, Nishikori was defeated by Federer, who went on to win the tournament for a seventh time.
ROGER FEDERER
Federer faces ace server in Halle semifinals
Defending seven-time champion Roger Federer on Friday reached the Halle semifinals where he will face Croatian giant Ivo Karlovic who fired a record-breaking 45 aces in his last-eight clash.
ROGER FEDERER
Is this the end of the road for Swiss tennis legend Roger Federer?
Roger Federer is talking optimistically about returning to his "highest level" after knee surgery, but does tennis have to start adjusting to a future without the Swiss star?
The 20-time Grand Slam winner announced on Wednesday that he would be sidelined until 2021 after his second operation in a matter of months.
Federer remains upbeat, tweeting: “I plan to take the necessary time to be 100 percent ready to play at my highest level.”
In some ways 2020 is a good season to miss after the coronavirus ravaged the tennis schedule. Writing Federer off in the past has proved dangerous.
He returned from a six-month injury lay-off to claim the Australian Open in 2017, winning his eighth Wimbledon crown later that year.
But he will be 40 in 2021 and is now heading into uncharted territory.
Despite his groaning trophy cabinet, there are two factors that will motivate Federer to keep going — the risk of losing his grip on the men's Grand Slam title record and a missing Olympics singles gold medal.
Rafael Nadal has 19 majors, just one shy of Federer's mark and Djokovic has 17.
Spain's Nadal will be fancied to draw level with Federer at the French Open, rescheduled for September, while few would bet against Djokovic winning in New York weeks earlier.
In April, Federer said he was “devastated” when Wimbledon was cancelled for the first time since World War II. Last year he fell agonisingly short at the All England Club, failing to convert two championship points on his own serve against Djokovic.
The Wimbledon grass probably remains his best chance of adding to his Grand Slam collection — he has not won the US Open since 2008 and his only title at Roland Garros came in 2009.
Even though Federer has slipped from the very pinnacle of the game, he is still a major threat to Nadal and Djokovic.
'Golden' ambitions
Last year, the world number four had a 53-10 win-loss record and he reached the semi-finals at the Australian Open in January in his only tournament this year.
Federer, who is still six ATP titles short of Jimmy Connors' all-time record of 109, has one glaring omission from his CV — the Olympic title.
The Swiss won doubles gold in Beijing in 2008 with compatriot Stan Wawrinka but lost in the singles final to Andy Murray in London four years later.
The postponed Tokyo Games will almost certainly be Federer's last opportunity to complete a career “golden” Grand Slam — he will turn 40 on the day of the closing ceremony next year.
Tennis will feel the loss of the elegant Federer keenly when he walks off the court for the last time.
Djokovic and Nadal have been the dominant forces in recent years but the Swiss remains the biggest draw and last month topped Forbes' list of the world's highest-earning athletes.
His last appearance on court was in front of nearly 52,000 fans — touted by organisers as a world record for tennis — at a charity match against Nadal in Cape Town in February.
Federer is nearly always the crowd favourite wherever he plays and has proved a perfect ambassador for the sport since he won his first Grand Slam title in 2003.
He certainly expects to be back and competitive next year.
“I will be missing my fans and the tour dearly but I will look forward to seeing everyone back on tour at the start of the 2021 season,” he tweeted.
The avalanche of support from his adoring fans showed they would miss him too, but they will have to get used to a time when he is gone for good.
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