SINGAPORE: Maria Sharapova made light of her long injury layoff by advancing to the last four of the WTA Finals with an unbeaten record in round robin play after the Russian breezed past Flavia Pennetta 7-5 6-1 on Thursday.

The victory also ended the career of the 33-year-old Italian, who announced this season would be her last after her stunning US Open victory last month, and booked a surprise passage to the semi-finals for Agnieszka Radwanska.

Radwanska had earlier battled past Simona Halep 7-6(5) 6-1 in their last Red Group match, with the Pole just two points away from elimination at one stage and only able to progress if Sharapova beat Pennetta in straight sets.

Perhaps aware that a single set was all she needed to extend her career by at least one more match, Pennetta started quickly, breaking in the first game against an opponent who had not completed a match since July before arriving in Singapore.

The Italian could not pull further away, however, and handed back the advantage in the eighth game, unable to find a first serve and double-faulting twice to gift Sharapova a break to get back to 4-4.

The pair continued on serve until the 12th game and with the Russian able to tee off on the Italian's modest serve knowing she had the safety net of a tiebreak, her relentless pressure told on Pennetta, who conceded the game and the set meekly.

The Russian now had the momentum and broke early in the second set through a combination of her typical booming forehands and recently developed sliced backhand chips and drop shots to surge 3-0 clear.

Pennetta staved off more break points to stop the rot as Sharapova was threatening to send her into retirement with a dreaded 'bagel' but the reprieve was brief as the Russian sent down her seventh ace on the way to winning the last three games.

SURPRISE BREAK

Radwanska had lost her first two matches and few would have given the fifth seed a chance of upsetting the highest-ranked player in the tournament after Serena Williams' withdrawal, especially when she took to the court with strapping around her right thigh and shoulder.

Her hopes appeared even more remote when the world number two started quickly, breaking in the first game and threatening to repeat the feat several times before Radwanska somehow regained parity with a surprise break in the 10th game.

Still deadlocked after 12 enthralling games, the opener went to a tiebreak and when the players changed ends with the Romanian 5-1 ahead, it appeared Halep would secure the set that would end Radwanska's interest in the tournament.

Her opponent had other ideas though, and threw herself around the court to reel off six straight points and seal the opener when Halep chipped wide at the end of a thrilling rally that had the crowd on their feet in appreciation.

The loss of the set had a demoralising effect on Halep and the Romanian was soon transformed into a helpless spectator as Radwanska stormed through the second set in 30 minutes, breaking three times as she chalked up her 500th win on the WTA Tour.

Radwanska, Halep and Pennetta all ended round robin play with 1-2 records but the Pole advanced after she won three sets in her matches while the others could only manage two.