Angelique Kerber has now 61 matches, more than anyone on the women's tennis tour.

Story highlights

Kerber has won both group matches

Keys beats Cibulkova to stay in contention

WTA Finals feature the top eight players

London CNN  — 

Angelique Kerber put aside her understandable end-of-season fatigue as she beat Simona Halep to close in on her first WTA semifinals spot.

Kerber struggled in her opening group match on Sunday, getting past Slovak Dominika Cibulkova in a tough three-setter.

But up against former French Open finalist Simona Halep on Tuesday, the German showed why she is the world No 1 as she wore down the light-footed Romanian with typically aggressive baseline play, 6-4, 6-2 to clinch her 61st match win of the season.

“She’s always a tough opponent, and I’m very happy to have my second match win here,” Kerber said in a courtside interview after beating the Romanian for the fifth time in eight matches.

READ: Angelique Kerber’s narrow escape on day one

Keys overpowers Cibulkova

The WTA Finals feature a round-robin format, with the top eight players in the rankings divided into two groups. Each competitor faces the three others in the group, with the top two in each section qualifying for the knockout semi-finals.

In the other match in the red group, Madison Keys posted her first ever WTA Finals victory as she blasted 26 winners past Dominika Cibulkova to win 6-1, 6-4 in just over an hour.

The 21-year-old American, who never faced a break point, remained in contention in the group after losing her opening match to Halep in straight sets. She’ll face Kerber in her final group match.

Breakthrough

It has been quite the year for Kerber, who took over the No. 1 ranking from 22-time grand slam singles winner Serena Williams after securing her first US Open title last month. Kerber also won the Australian Open title at the start of the year and reached the finals of Wimbledon.

Although Kerber laid the foundations for her breakthrough year with grueling workouts during the off-season, the biggest improvement has been made on the mental side.

“I have more confidence right now,” Kerber told reporters before the tournament started. “I know how to play the big tournaments also mentally, how to win really tough matches in tough situations, like playing finals in big tournaments, big stages. So this changed a lot [for me] as a player.”

READ: How a smart court helped Simona Halep improve her serve

Maturity

Kerber’s newfound maturity showed in the first set.

After leading 3-1, Kerber suddenly found herself down 4-3 and faced with a break point as Halep put her under pressure from the back of the court. The German remained cool, fending off the break point and winning the next five games as she hustled, retrieved and fought for every shot.

Although the left-handed Kerber is known for her strong defensive skills and athleticism, she surprised Halep with a crafty sliced backhand drop shot to set up a break point leading 2-1 in the second set.

“Swing out of your shoes,” Halep’s coach, Darren Cahill, told his protégé as she trailed 4-1 in the second set.

Halep did, but it wasn’t enough as Kerber closed out the match on her third match point.

READ: Svetlana Kuznetsova gives herself a mid-match hair cut

Keys comes to grips with surface

“Finally got the nerves out after the first match,” Keys said in a court-side interview after beating Cibulkova of Slovakia for the fourth time in a row.

After dropping her opening match, the big-serving Keys said she’d struggled to adjust to the slow surface in Singapore.

“I just had to kind of take a step back and not rush and know a lot of balls were going to come back into play,” Keys said. “I think I was just more prepared for that tonight.”

READ: Halep and Keys on what it takes to become a top tennis player