Novak Djokovic moved a step closer to becoming the first men's player in 52 years to complete a calendar-year singles Grand Slam on Wednesday, though the world No 1 was made to work for his US Open quarter-final win over Matteo Berrettini.
In a repeat of July's Wimbledon final, top-ranked Djokovic eliminated sixth seed Berrettini 5-7, 6-2, 6-2, 6-3 at Arthur Ashe Stadium to stretch his Grand Slam win streak to 26 matches.
"I think it was a great performance overall," Djokovic said, calling his last three sets "the best three sets I've played in the tournament."
Next up for Djokovic is a Friday semi-final showdown against Olympic champion Alexander Zverev, who beat Djokovic in the semi-finals in Tokyo.
"He's in tremendous form. He has been winning a lot," Djokovic said of Zverev. "He's one of the best players in the world right now and I'm pumped. The bigger the challenge, the more glory in overcoming it."
Djokovic is trying to become the first man to win all four major titles in the same year since Rod Laver in 1969, but he wasn't ready ponder the glory of that challenge just yet.
"Only focusing on the next match. Don't ask me about history," Djokovic said. "I'm thinking only about the next match. Step by step."
The 34-year-old Serb also seeks his fourth US Open title and men's record 21st career Slam trophy, which would lift him one above the mark he shares with Roger Federer and Rafael Nadal, both absent with injuries.
Zverev denied Djokovic's bid for a "Golden Slam" sweep of all four major titles plus Olympic gold, but Djokovic leads their all-time rivalry 6-3.
"He's the best player in the world. He's very difficult to beat," Zverev said. "I was the first player to beat him in a very big match this year. That does give you something."
Fourth-seed Zverev extended his winning streak to 16 matches by easing past South African Lloyd Harris 7-6, 6-3, 6-4.
"He was serving incredible, in the first set especially," Zverev said. "Somehow was able to win that tie-breaker and that loosened me up.
"I'm really happy to get through in three."
Zverev broke Harris for a 2-0 lead in the second set and held serve from there. In the third, Zverev broke Harris in the first and third games and won after two hours and six minutes with his 21st ace of the match.
"The level wasn't quite where it needed to be," Harris said. "A little disappointing in that regard. Nonetheless, still a lot of positives to take from the week."
The best prior Slam run for Harris was to the third round of this year's Australian Open.