World number four Juan Martin del Potro could miss the rest of the season after fracturing his right kneecap in a heavy fall at the Shanghai Masters.

The 30-year-old Argentine retired from his last-16 match against eventual finalist Borna Coric on Thursday after falling during the first set.

Former US Open champion Del Potro, who has his leg in a splint, said: “It’s a very difficult moment. I feel very sad.

“It’s a hard blow that leaves me without strength.”

Del Potro had been expected to play in the season-ending ATP World Tour Finals in London in November.

“It’s very difficult for me to think about recovery again, I did not expect this to happen,” he said.

Del Potro has been plagued by injuries since winning his only Grand Slam in 2009.

Serb Novak Djokovic defeated Croat Coric 6-3 6-4 in Sunday’s final in Shanghai.

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World number eight Sloane Stephens has qualified for the season-ending WTA Finals for the first time.

The 25-year-old American became the sixth player to qualify for the event in Singapore, which begins on 22 October.

Her place was confirmed after world number five Elina Svitolina confirmed she will not play an event this week.

The final two spots will be taken by either Svitolina, Karolina Pliskova or Kiki Bertens.

Already qualified are world number one Simona Halep, Wimbledon champion Angelique Kerber, US Open winner Naomi Osaka, Petra Kvitova and Australian Open champion Caroline Wozniacki.

The last two positions will be decided at this week’s Kremlin Cup in Moscow where both Pliskova and Bertens are playing.

Garcia takes Tianjin Open title

Meanwhile, Caroline Garcia won her first title of the year as the French player defeated Pliskova to edge a nervy Tianjin Open final in China on Sunday.

She battled through a narrow first set before defeating the Czech top seed 7-6 (9-7) 6-3 for the sixth title of her career.

Novak Djokovic continued his superb form by beating Croatia’s Borna Coric in the final of the Shanghai Masters.

The second seed, 31, beat 21-year-old Coric 6-3 6-4 in 97 minutes to claim his fourth Shanghai title.

Coric, the 13th seed, upset Roger Federer in Saturday’s semi-finals to reach his first Masters 1000 final.

Serb Djokovic has won 18 consecutive matches since losing at the Rogers Cup on 10 August, and will replace Federer as world number two on Monday.

The 14-time Grand Slam champion cruised through the first set, dropping just four points on his serve.

Coric recovered from 3-0 down in the second to earn a break point at 3-2 before saving three championship points to break Djokovic and make it 5-4.

But Djokovic won his 26th straight set to clinch his fourth title of 2018 and go just 35 points behind Rafael Nadal in the ATP Race for London.

Djokovic’s winning run includes victory at the Cincinnati Masters in August, when he became the first player to win all nine Masters 1000 tournaments.

• Murray & Soares beaten in Shanghai final
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Jamie Murray and Bruno Soares were beaten in straight sets by Lukasz Kubot and Marcelo Melo in the Shanghai Masters doubles final.

The British-Brazilian duo, who this week qualified for the ATP Finals in November, lost 6-4 6-2 to the Polish-Brazilian team.

Kubot and Melo went one better than last year when they were beaten finalists.

Murray and Soares have won three titles together this season.