NEW YORK • Serena Williams will be wary of another US Open semi-final slip-up when she takes to the Arthur Ashe Stadium court this morning (Singapore time) to face 10th seed Karolina Pliskova.

A year ago, the American was on the cusp of a rare calendar slam when unseeded Italian Roberta Vinci, a 300-to-1 long shot to reach the final, recorded one of the biggest upsets in Grand Slam tennis.

It appears that Williams has learnt her lesson. The world No. 1 has been playing with unusual intensity at Flushing Meadows as she zeroes in on a record seventh US Open title in the professional era.

Standing in her way is big-hitting Pliskova, who has led the Women's Tennis Association Tour in aces the last two seasons and is one of the few players capable of going toe-to-toe with Williams.

Pliskova came into the tournament with a Tour-leading 407 aces this year - 147 more than second-placed Williams with 260.

"There will be a lot of aces in that match I think," said Williams, who fired 18 aces in her 6-2, 4-6, 6-3 quarter-final win over Simona Halep, which stretched her record for Grand Slam match wins to 309.

"I haven't played against many big servers recently but she places her serve really well. She definitely gets some speed on it but her placement is really, really nice."

There is no calendar Slam up for grabs this year but Williams would be the greatest Major champion in the Open Era if she claims a record 23rd title. While Williams will compete in her 33rd Major semi-final, the match is uncharted territory for Pliskova, who arrived as the only top-20 player that has not reached the fourth round at one of the four Grand Slam events.

The Czech won the biggest title of her career last month on the hard courts of Cincinnati, where she upset Angelique Kerber in the final.

But she acknowledged that Williams will be another matter.

Pliskova said: "She's a big hitter and she can have 50 winners and you cannot do much about it."

REUTERS, AGENCE FRANCE-PRESSE