Halle Westfalen - Roger Federer said he was hoping for some easier victories after a battling 6-3, 4-6, 6-4 win over Spain's Roberto Bautista Agut to reach the last four of the ATP event in Halle for the 15th time.
World number three Federer is chasing a record-extending tenth title on the Halle grass as he warms up for Wimbledon next month, but has had to scrap to stay in the tournament this week.
He prevailed in a gruelling three-set encounter with Jo-Wilfried Tsonga on Thursday, and was taken the distance again by Bautista Agut in Friday's quarter-final.
"My body is reacting well to the grass, so I would prefer the matches to be shorter," said Federer.
"But if not, it's no problem. I have enough time to recover before Wimbledon."
The Swiss next faces Andy Murray's Wimbledon doubles partner Pierre-Hugues Herbert in Saturday's semi-finals.
"I know Pierre-Hugues well. He returns well and looks to get to the net, which makes him a great grass player," said Federer.
The 20-time Grand Slam champion has now reached the last four in each of his last 15 appearances at Halle since losing to Pat Rafter in the 2001 quarters.
The 37-year-old will be aiming for his ninth Wimbledon triumph in July, having seen Rafael Nadal cut his lead in the all-time Grand Slam title winners' list to two with victory at the French Open earlier this month, when the Spaniard beat Federer in the semi-finals.
Herbert won the first set of his quarter-final 7-5 against reigning champion Borna Coric, before the Croatian retired with a back problem.
The 28-year-old Frenchman said that he felt an "unbelievable energy" going into his first-ever meeting with Federer.
"It will be unbelievable for me to face Roger. I have played against Nadal, against Djokovic, against Murray, but never against him," said Herbert.
Earlier on Friday, world No 5 Alexander Zverev suffered a surprise 6-3, 1-6, 6-7 (3/7) quarter-final defeat by Belgium's David Goffin.
Home favourite Zverev was playing with a swollen knee after slipping and hurting himself in the first round, and the Halle crowd watched on in dismay as he struggled to find his rhythm against world number 33 Goffin.
"You can look at things negatively or positively. I played three good matches here, and it's not as if he blew me away today," said Zverev.
The defeat compounds a disappointing grass season so far for the German, who suffered another early exit on home soil in Stuttgart last week at the hands of compatriot Dustin Brown.
The 22-year-old is hoping to rediscover his form and fitness in time for the beginning of Wimbledon on July 1.
"I am not at 100 percent, but I think I'll be ok," he said.
Goffin will face Italian talent Matteo Berrettini in the semi-finals.
Berrettini continued his excellent 2019 form with a 6-2, 7-6 (7/4) victory over Russian third seed Karen Khachanov.
Berrettini, 23, won his third ATP title of the season in Stuttgart last week, and has soared up to 22 in the world rankings this year.