The eyes of the tennis world are very much on Serena Williams during this year’s US Open, and only two opponents now stand between her and the historic achievement of winning all four Grand Slam tournaments in a calendar year.

Roberta Vinci is the first of those opponents, and the Italian is in uncharted territory, having never reached a Grand Slam semifinal in singles before. She has also never beaten Serena before, and she has not even won a set, so it will truly be a match of firsts if she upsets the odds and defeats the world No. 1 on her home turf.

Vinci, 32, has battled her way to the last four with three-set victories over Denisa Allertova, Mariana Duque Marino, and Kristina Mladenovic, following a straight-sets win over Vania King in round one. She also benefited from a walkover in the fourth round due to Eugenie Bouchard’s extremely unfortunate withdrawal with a concussion.

But at no point has the Italian demonstrated sufficient form to suggest she may be able to overcome Serena. The American has been in sparkling form since she lost the first set to Bethanie Mattek-Sands in round three. That brief setback seemed to ignite the fire in her, and Serena played well to stage a successful comeback in that match, then brilliantly during her straight-sets victory over rising star and heir-apparent Madison Keys, and even better during a near-perfect opening set against her sister Venus. Although her level dropped alarmingly in the second set against Venus, Serena recovered well to take the third set with ease, and she looks well situated to beat Vinci and win the tournament.

Simona Halep represents the biggest threat to Serena’s calendar Grand Slam ambitions. The Romanian world No. 2 is playing arguably the best tennis of her life, and she was absolutely sensational at times during her quarterfinal win over Victoria Azarenka – a result many pundits found surprising. They should not have underestimated Halep, however, since the Romanian has been in superb form during the US Open Series, and she seems to grow stronger mentally with every match she plays. She served and returned better than Azarenka, and her superiority in those key areas eventually told on the scoreline as she won a tight match 6-3 3-6 6-4.

It could have been a very different quarterfinal had Sabine Lisicki been Azarenka’s opponent. The German had several chances to seize the initiative in the second set of her fourth-round match with Halep, but she failed to take any of them, and the Romanian survived and played much better in the third set to win it 6-2. She had no such trouble with her first three opponents in the 2015 US Open, dispatching all of them - Marina Erakovic, Kateryna Bondarenko, and Shelby Rogers – in straight sets.

Standing in the way of her first US Open final is another Italian, Flavia Pennetta, whose run to the semifinal is one of the most impressive achievements of her career. Pennetta has beaten Samantha Stosur, Petra Kvitova, and an in-form Petra Cetkovska along the way. It is a far less surprising run than that of her compatriot Vinci, however, since Pennetta reached the same stage two years ago, and she has reached the quarterfinals or better at the US Open on five occasions. She actually beat Halep in straight sets in the fourth round in 2013, but the Romanian has improved out of sight since then, and she defeated Pennetta in straight sets in Miami earlier this year.

Predicting the outcome of this second semifinal might be difficult if not for Halep’s excellent form. She simply must be considered the heavy favorite to beat Pennetta, despite the obvious threat the Italian possesses. It could go to three sets, but the Romanian will be determined to win and will probably do exactly that. If she does, the 2015 US Open will get the final it deserves – a titanic clash between two tennis players who fully justify their status as world No. 1 and No. 2: Serena Williams and Simona Halep.