Halep, Muguruza among those looking for more consistency

— -- Serena Williams had the best season on the women's tour, no question. With three Grand Slam titles and another major semifinal among five other tournament victories, she was No. 1 by a long way.

But when it comes to who was the second-best player in 2015, there's no simple answer.

Despite all the contenders, it's tough to see who was best of the rest. Numerically, it's Simona Halep, who finished No. 2. But her biggest title was the BNP Paribas Open at Indian Wells, and she fell hastily in the only Grand Slam semifinal she reached all year.

Looking at the majors, the only player other than Williams to win a title was Flavia Pennetta, the surprise US Open champion. But apart from that prized run, the Italian didn't get further than the quarters at any other event -- and has subsequently retired.

"Players fluctuate so much more than in my time," said Martina Navratilova, one of several former Grand Slam champions addressing the media at an all-access WTA Finals media event last week. "Yes, there is much greater depth, no doubt. But still doesn't explain the level of fluctuation."

As far as beating Williams, only three players accomplished this feat in 2015. Roberta Vinci was the only one to get a win when it counts most, at the majors: She stunned Williams with her unorthodox game in the US Open semifinals. Petra Kvitova snagged a win against Williams on her way to winning the Mutual Masters at Madrid, and Belinda Bencic also defeated an injured Williams at the Rogers Cup en route to the title.

In terms of consistency, not one player after Williams had a significant edge.

Breaking it down into parts of the season doesn't help much. Maria Sharapova reached the Australian Open final and Wimbledon semifinal, falling to Williams both times, and was a consistent presence on clay. That made Sharapova one of the best players during the first half of the year, but she barely played during the second half.

Halep went 23-3 until the clay season but was just 26-14 the rest of the season.

Garbine Muguruza, on the other hand, made a push in the second half by reaching the Wimbledon final and Wuhan semifinals and then by winning the Beijing Open. However, the Spaniard, 22, who jumped to No. 3 by year's end, suffered a long drought during the summer hard-court season.

Agnieszka Radwanska started the season slowly but finished on a tear, winning three titles including the WTA Finals at Singapore on Sunday.

Even by surface, there's a split: Halep had 41 wins on hard courts, leading even Williams. Lucie Safarova reached the final of the French Open in a clay-court season that saw several winners. Radwanska performed well on grass, and Angelique Kerber won tournaments on all surfaces -- but not events that carried any significant weight.

Former Wimbledon champion Marion Bartoli argued that the byproducts of success make it harder to sustain it, especially when the field is so competitive.

"A lot more players are getting a lot more professional and a lot more players traveling with a huge structure," the Frenchwoman told the media. "Not only the top 10 anymore who have trainers and coaches and who are really getting into the best shape they can possibly be.

"As a player, when you just arrive on the scene and have these great results. When you have to defend those, the margin between winning and losing becomes tinier and tinier."

The WTA Finals should have been the perfect way to settle who is next-best at the moment, except it wasn't. The tournament featured the best eight players other than Williams. But most of the players collected more losses than wins -- and even winning didn't guarantee success.

Sharapova and Muguruza went 3-0 during the round-robin stage, but both fell in the semifinals. The finalists, Radwanska and Kvitova, both went 1-2 in the round robin but somehow found themselves playing for the title, which Radwanska won in three sets.

"What's interesting is just the consistency level," said Tennis Channel analyst Tracy Austin in Singapore. "So many of these players today, I just find the consistency level is so up and down. Is it more depth? Is it they feel more pressure? It is just an interesting time."

Both pressure and playing style could be part of the explanation.

"It has to do with mentality, but I also think if you're going to get to the top 10 or top 20, you have to be mentally tough anyway," 18-time Grand Slam champion and ESPN analyst Chris Evert told the media. "I think training-wise, when I look, the women are all in great shape, great condition."

Evert noted that there were a lot of injuries and withdrawals "because the year seems to be a little too long and challenging for a lot of the women. But for the most part, I just think there is more of an even playing field because of the fact that the women don't have dominating serves."

If it's tough to pick who's No. 2 at the moment, it's even more difficult to envision who might solidify herself in that position in the coming months. Most of the top 30 players are capable of a big run. Fewer, however, seem capable of keeping up consistently stellar performances, let alone keeping up with Williams.

They might not be giving her much competition at the moment, but at least they're providing plenty of competition for each other.