SHENZHEN, China – Petra Kvitova had some big moments in 2015 – three Premier titles at Sydney, Madrid and New Haven, her first US Open quarterfinal and, for a brief period, getting back to No.2.

But there was something else that’ll go down in the record books – a shocking win over the World No.1.

“She went for broke on every serve, every return – I hit some second serves, 170, 175, and she would just hit them for winners,” Serena Williams said after falling to Kvitova in the Madrid semifinals.

“For sure I’m happy I won today,” Kvitova said after her 6-2, 6-3 victory in the Spanish capital.

“I had never beaten Serena before, so this is a really special moment for me, of course.”

With that victory, Kvitova became the only player to beat Williams in straight sets in 2015 – and with that, could the big-hitting Czech, a former World No.2, be the World No.1’s biggest threat in 2016?

She certainly knows how to bring her best on the biggest stages – also Williams’ favorite stages. Kvitova has two Grand Slam titles to her name, Wimbledon in 2011 and 2014, and she’s been to the semis or better at the Australian Open and French Open, and the quarterfinals of the US Open as well.

The World No.6 is also an impressive 17-6 in WTA finals – more importantly, 2-0 in Grand Slam finals.

“I don’t really like the beginning of the tournaments, for sure, but that’s how it is – it’s probably normal that I’m improving my game with each match in a tournament,” Kvitova said. “And then when the finish line comes, I’m feeling better about my game. Unfortunately I can’t just play finals, which is sad!”

Speaking of finals, Kvitova suffered a rare loss in a title match at the WTA Finals in Singapore this year – to Agnieszka Radwanska – though she did score a big win over Maria Sharapova in the semis.

She was asked afterwards what she was hoping to work on in the off-season. “The main thing I want to do is improve my health,” she said. “I have a few weeks for that, so I hope it’s going to help me, and my main goal is to be prepared and healthy for the next season. For sure that’s my biggest goal.”

Kvitova will kick off her 2016 season at the Shenzhen Open in Shenzhen, China, an International-level event that boasts twice the usual International prize money – $500,000. Radwanska will be there too.

“I’m very happy to be going back to Shenzhen,” Kvitova commented. “Last year I had a very nice time. Everyone was very kind to me, and it was also very nice to see my good friend Li Na when I was there.

“I look forward to playing there again, seeing Li Na again and hopefully winning the title this time.”