I'll be better for Halep test - Serena

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World number one Serena Williams believes her fiercely-contested three-set win over Simona Halep in the US Open quarter-finals will hold her in good stead in her quest for a 23rd grand-slam title.

Williams booked her spot in the semi-finals on Wednesday after withstanding fifth seed Halep 6-2 4-6 6-3.

The 34-year-old American had not dropped serve throughout the year's final major up until her matchup against Halep, with 43 unforced errors threatening to derail her in New York.

But Williams feels she will be better for the two-hour, 14-minute battle at Arthur Ashe Stadium.

"I feel great. I'm glad I got tested," Williams told reporters post-match.

"The best part of it is I feel like - I mean, I think her level really picked up in the second, but I had some opportunities that I didn't take.

"So what I gather from that is I really could have played better in that second set and maybe had an opportunity to win in straights.

"I think if anything, that's the biggest silver lining I take."

Williams, who will meet 10th seed Karolina Pliskova in the final four, added: "I have been really stressed out in a few of the matches, but I felt pretty good in most of the rounds.

"Honestly I feel okay today. I feel like I could have played a lot better. I think that makes me really calm, is the fact that I know I can play so much better and I have to, I know that. I think everyone here knows I can play better."