When it was all done, when two hours and 14 minutes of glorious struggle was over, Serena Williams applauded Simona Halep as she picked up her gear and walked off Arthur Ashe Stadium Court for the last time in this U.S. Open.

Williams had just completed a 6-2, 4-6, 6-3 quarterfinal victory Wednesday night in which Halep had tested every aspect of her game. Sure, Williams served 18 aces as she usually does, but when her serve came back, Williams knew that Halep was thoroughly in the match.

Halep had been far more than a worthy opponent. She had been a downright pain in the very best sense. She pressured Williams constantly, and was the very equal in long rallies of scintillating groundstrokes that tightened the screws of emotion throughout the stadium with every hit.

Williams broke Halep to win the first set, then held serve in the first game of the second. Game 2 is where Halep made her stand. It was a monumental tussle, going to seven deuces after Halep was initially down 0-40. She saved all those break points and four others, running down every ball, firing winners and hitting enough jam serves to keep Williams off balance.

It was the impetus to her second-set win, propelled by breaking Williams in the third game. Halep then had to save four break points in the 10th game of the set to close it out. In all Halep saved an impressive 16 break points.

Williams knew she had been in a dogfight and tried not to let frustration set in when she couldn’t convert the breaks.

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“I’m glad I had the break points as opposed to not having them,” Williams said. “That’s the only thing I can do, and hopefully I can start getting more.”

After not facing a seeded player in the first four rounds, Williams found herself up against the No. 5 seed, a significant upgrade. “I think that she obviously plays well,” Williams said. “Yeah, so it was definitely a jump .”

Halep, who lost to champion Flavia Pennetta in the Open semfinals last year, was disappointed but not downcast. She had a break point against Williams to start the third set, but failure to convert ultimately led to failure to win.

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“I played well. I could play better in those moments when I had chances,” Halep said. “But I think the level was pretty high. I’m OK with the way that I was fighting till the end. It’s something normal now for me, so it’s a good thing.”

Next up for Williams in the semifinals Thursday night is Karolina Pliskova, who beat Ana Konjuh on Wednesday.

Pliskova eliminated sister Venus in a taut and tantalizing three-setter in the round of 16. Serena and Pliskova have met once with Serena winning in two sets at Stanford in 2014.

“I was a completely different player at that time, so I improved a lot and obviously she probably as well,” Pliskova said. “She’s a big hitter and she can have 50 winners and you cannot do much about it. But I still am gonna hope that I can get my chance and be the one who is playing aggressive.”

Pliskova has a big serve, though not the equal of Williams. “I haven’t played against many big servers recently, but she places her serve really well, I think,” Williams said “She definitely gets some speed on it, but her placement is really, really nice.”

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But Pliskova will be in Ashe Stadium, Serena’s place, Thursday night.