Story highlights

Top seed Simona Halep exits WTA Finals

Romanian ousted by Agnieszka Radwanska

Maria Sharapova advances to semifinals

Russian ends Flavia Pennetta's career

CNN  — 

At last year’s WTA Finals, Simona Halep handed Serena Williams the joint worst loss of her career and reached the title match to confirm her status as one of the rising stars in women’s tennis.

This week in the Singapore showpiece, things have not gone to plan for the Romanian.

With world No. 1 Williams absent as she chooses to recover from a draining campaign, No. 2 Halep entered the end-of-season event as the top-ranked player.

And the 24-year-old began the tournament well, defeating U.S. Open champion Flavia Pennetta in straight sets.

Unfortunately for Halep, it went downhill from there.

She lost in straight sets to Maria Sharapova on Tuesday and then Thursday was ousted in the group stage after falling to Agnieszka Radwanska 7-6 (7-5) 6-1.

It was a battle of two of the most popular women on tour – Halep was the most clicked player on the WTA’s website last year while Radwanska has been voted as Fan Favorite on the circuit for four consecutive seasons.

The result meant Sharapova reached the semifinals without having to hit a ball against Pennetta later Thursday – and when the Russian overcame the Italian in straight sets 7-5 6-1, Radwanska joined her in advancing.

Halep emerged as a grand slam contender in 2014, narrowly losing the French Open final to Sharapova in a three-set thriller. But this year she was upset at the Australian Open in the quarterfinals, exited in the second round in Paris and first round at Wimbledon. Despite making the semifinals in New York, there, too, she was upset by Pennetta.

The turning point in Halep’s defeat to Poland’s 2012 Wimbledon finalist was easy to spot. Halep led 5-1 in the tiebreak, only to drop the ensuing six points.

Radwanska struck two fine returns to gain a 6-5 advantage in the tiebreak before winning a highlight-reel rally that concluded with Halep’s volley going out.

Out, too, went Halep’s chance of winning the Red Group clash.

“I was done,” Halep told reporters. “No energy anymore. I was tired.

“I felt that I lost the chance to win the first set and probably I lost the chance to win the match in that moment.

“My coach was telling me many things, but I couldn’t hear because I was done and I was very nervous there.”

Radwanska isn’t done, thanks to her win and Pennetta not claiming a set versus Sharapova, who topped the group with a perfect 3-0 record. Pennetta led 4-2 in the first.

Sharapova has “surprised” herself with her performances in Singapore, since the five-time grand slam champion hadn’t completed a match – not including a retirement – since Wimbledon. A leg injury hampered the 28-year-old.

“I’m actually … a little bit surprised that I’ve been able to win three matches, as physical as some of those matches were,” Sharapova told reporters.

“I think knowing that I qualified allowed me to be a little bit more aggressive today. I thought I played quite aggressively in the second set and had a good ratio of winners and unforced errors, was quite solid. Served well.”

Winning the WTA Finals would have been the best possible sendoff for Pennetta, who said after winning the U.S. Open that she would retire when the season finished. Pennetta didn’t linger when Thursday’s match wrapped up, quickly walking off and waving to the crowd.

“I think to have the last match against Maria, was amazing to play such a good champion,” said Pennetta. “Was a good way also to say goodbye, because when you lose against such a good player there is not too many things to say.

“Right now I feel?like it’s not my last match. I feel normal completely. I don’t know why. Maybe in few days I will feel more the difference. Right now it’s so far so good.”

All four players in the White Group remain in contention. Wimbledon finalist Garbine Muguruza leads with a 2-0 record, followed by Petra Kvitova (1-1) and Angelique Kerber (1-1). Second seed Muguruza faces two-time Wimbledon champion Kvitova on Friday, with Kerber playing French Open finalist Lucie Safarova (0-2).

Read: ‘Emotional’ Kvitova beats friend

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