In a year when she won her first singles Grand Slam title, bagged her first Olympic singles medal and reached the pinnacle of women's tennis, Angelique Kerber remains on course for another breakthrough - reaching the last four of the WTA Finals for the first time.

The German, who will also finish the year as world No. 1 for the first time, held off a gutsy Simona Halep yesterday to move closer to the semi-finals of the BNP Paribas WTA Finals Singapore presented by SC Global.

The 28-year-old failed to make it past the round-robin stage in her previous three appearances (2012, 2013, 2015) and had compiled a mediocre 2-7 win-loss record.

She doubled her tally of wins last night with her second victory at the US$7 million (S$9.75 million) season-finale. But while her 6-4, 6-2 effort over Halep was 55 minutes shorter than her three-set opening match against Dominika Cibulkova, the match was more demanding than the score suggested.

Said Kerber: "Against Simona, you never know. She's fighting until the last point; it doesn't matter what the score is - she's playing point by point.

"It was not so easy mentally also, because what I know about her is that she's going for it."

Aggressive play from Kerber at the start of the match saw her break the Romanian in the opening game, but the latter forced errors off the Kerber racket to get back on serve.

Kerber responded with a break to regain her lead at 5-4 and successfully served out the set.

She started the second set in identical fashion, breaking Halep's serve before winning her fifth game in a row to open up a 2-0 lead.

Kerber displayed more of the grit that had dug her out of the trenches during her first match on Sunday.

Even as Halep's clever net play and searing down-the-line winners saved two match points, Kerber resisted, pushing the ball across the net repeatedly until an errant Halep backhand sealed the win.

It brought Kerber's head-to-head record with Halep to 4-4, with all of Kerber's wins coming this year.

With victories in the Australian Open and US Open, she is a woman transformed and aptly, was presented with a Tiffany No. 1 ring and the WTA Year-End No. 1 trophy after her match.

She said: "I have much more confidence right now, because I know how to win very big matches, and tight matches.

"I know what to do to go for it and take the game in my hands... I won so many good matches this year. I won two Grand Slams, so that's why I believe much more in myself right now.

"I improved so much in my mentality as well and my positive body language. I think that's also a change... I think this gives me a lot of confidence as well when I have points like that and to stay positive and just going for it."

Halep gave credit to her opponent, but rued her missed chances in the match.

She said: "I didn't finish the important points and I missed a lot today.

"But (Kerber) played great tennis; she's very strong on her legs and she's moving great, so she deserved to win."

But far from feeling discouraged, the Romanian is confident going into her final match against Cibulkova, who remains winless after a 1-6, 4-6 loss to fellow debutante Madison Keys yesterday.

Said Halep: "She (Cibulkova) doesn't move as well as Kerber; I always feel I can finish the points when I play against her. I have to be aggressive, play my style and just be there for every point."

Kerber leads the Red Group and will guarantee top spot if she can collect a third straight win against Keys tomorrow at the Singapore Indoor Stadium.

WTA FINALS
Day 4: Singles round robin StarHub Ch203, 7.30pm