When Garbine Muguruza struck the lob that clinched the 2016 French Open title, she put the seal on a nine-month period featuring only one tournament win for Serena Williams. It is a remarkable fact, especially when you consider that the American won three Grand Slams and two Premier titles during the first eight months of 2015.

So what’s caused this change? Have Serena’s standards declined? Or have her opponents improved? The answer is a bit of both, and the combination has produced a significant shift in power in women’s tennis, which has seen Victoria Azarenka, Angelique Kerber, Agnieszka Radwanska, and Muguruza step into the ring as genuine contenders to overtake Serena’s status as the best in the world. Who could have imagined Serena’s 2015 US Open semifinal loss to an inspired Roberta Vinci would lead to all this?

Kerber’s rise is the most impressive. Before 2015, she was starting to look like a player who was very consistent but ultimately unable to make the necessary step up to become one of the best in the world. Losses in big matches were costing her, and this was never better illustrated than in 2014, when she lost every one of the four finals she reached. However, she turned that record around in 2015 by winning four Premier titles – a tally bettered only by Serena.

These performances provided ample evidence that Kerber was steadily improving and might reach the top, and then she showed everyone just how good she now is by beating Serena in a brilliant three-set match to win the 2016 Australian Open. Unfortunately for the likeable German, a slump followed, and decent form in Charleston followed by an impressive defense of her title in Stuttgart provided only a brief respite from her slump into inconsistency.

Kerber’s Stuttgart win raised hopes of success at Roland Garros, but poor showings in Madrid and Rome left her short of form going into her first-round match against big-hitting Dutch player Kiki Bertens, and she lost a rollercoaster match in three sets. After reaching the semifinals in Rome and negotiating a tricky first-round match with Anna Karolina Schmiedlova, Muguruza started to find her best form at Roland Garros, destroying former champion Svetlana Kuznetsova and former finalist Sam Stosur on her way to the final against Serena.

Once there, the Spaniard, 22, produced the best performance of her career so far to take down the world No. 1 in style. The relative closeness of the 7-5 6-4 scoreline was a testament to Serena’s fighting qualities because it was Muguruza who dictated the way the match went. It is up to the newly installed world No. 2 to show us exactly how many more Grand Slams she can win in the future, but her superb triumph at the French Open is extremely unlikely to be her last major title. Muguruza is far too good for that. She has a great chance of winning Wimbledon a few weeks from now and probably has her best years ahead of her. The question is whether she will be consistent like Serena, or sporadically brilliant like Petra Kvitova or Maria Sharapova?

In an interview she gave shortly after completing her stunning Sunshine Double with a victory in Miami, meanwhile, Victoria Azarenka appeared determined to conquer the clay. She said, “I've always been a favorite of proving people wrong. That's what motivates me a lot. When people say it's not my favorite surface, I'm going to work really hard to make sure it's my favorite surface.” The Belarussian is one of the most determined players ever to grace a tennis court, and it seemed plausible that she might achieve success at Roland Garros through sheer force of will. The reality was very different, as Azarenka only won two matches during the clay-court season and struggled at various times with injuries that eventually forced her to retire during her first-round match at the French Open. She has never played her best on grass either, but it would be foolish to suggest she has no chance of winning Wimbledon.

One woman who is totally at home on grass is Agnieszka Radwanska. She has appeared 10 times at Wimbledon and reached the final once, the semifinals twice, the quarterfinals twice, and the fourth round on three additional occasions. She also won the Eastbourne title in 2008 and reached the final there in 2015. These performances are not surprising when you consider how well-suited her game is to grass. Radwanska hits vicious low slices that don’t bounce up as they would on other surfaces, gets full value for her shots because defending is more difficult on grass, and uses her fantastic touch and great reflexes to play extremely effective shots at the net.

Furthermore, that run to the Eastbourne final last year (together with a semifinal showing at Nottingham the week before) triggered a spectacular return to form that enabled Radwanska to reach the Wimbledon semifinals before finishing the season in stunning style by winning three titles in the Asian swing, including the season-ending WTA Finals. Considering the talented Pole’s form has continued into 2016, and she will now be playing on her best surface, it is possible she could become the fourth successive first-time Grand Slam champion in women’s tennis at SW19 on July 9.