At first, second, third, and even fourth glance, this is a mismatch and a blowout in the making. Serena is 4-0 against the 43rd-ranked, 32-year-old Vinci, and the eight sets they’ve played have gone like this: 6-3, 6-4, 6-2, 6-1, 6-1, 6-3, 6-4, 6-3. In the past, Serena has had too much power for the finesse-oriented Italian. Should that be the end of the conversation? Probably. But as we know, strange things can always happen in sports. Despite the straight-set scoreline, Vinci played a quality match against Serena in Toronto last month, and gave her some trouble with her one-handed slice backhand and unusual soft-ball style. Serena hasn’t forgotten. “She played me really tough, and I didn’t expect that,” Serena said on Tuesday. “...so I’m not going to underestimate her. She played really well. She’s not in the semifinals of a Grand Slam for no reason.”

So Serena, as she usually is by this time in a major, is staying alert. After facing two big hitters in Madison Keys and her sister Venus, she knows she’ll have to shift gears against the light-hitting Vinci. And after maintaining a laser focus the last two rounds, it’s possible she could become frustrated if she starts to miss in this match. But she’ll win anyway. Winner: S. Williams

Tennis players need to have short memories, they say, and Halep demonstrated just that after her quarterfinal win on Wednesday. When she was asked for her thoughts on her semifinal opponent, Pennetta, Halep was happy to volunteer that she had played her earlier this year. It’s true, they faced off in Miami in April, with Halep winning in straight sets. What she didn’t mention was that this was her first win in four tries against the Italian. Two years ago, Pennetta eliminated Halep from the Open in the round of 16.

Despite her 1-3 head-to-head record, Halep is the favorite this time around. She’s ranked No. 2; Pennetta is No. 26. Halep is 23 years old; Pennetta is 33. Halep has been to a Grand Slam final, at the French Open last year, while Pennetta hasn’t. Halep has also had an excellent summer, and is coming off what may have been the most impressive mix of playing and competing of her career, in her three-set win over Victoria Azarenka in the quarters. But this match still feels like it could go either way. Halep is prone to ups and downs, and Pennetta, who saves her best tennis for New York, is nothing if not experienced. Both women will be playing without a rest day; if Halep is tired from her back-to-back three-setters, or looking ahead to a date with Serena Williams in the final, Flavia could steal it. Winner: Halep