China's Wu Di, a former ball boy in Shanghai, is ranked 230th and also used to be Nadal's practice partner. He broke Nadal's serve four times throughout the match, which the former world no.1 eventually won 6-4 6-4.

"As for the ATP Finals, I served as the ball boy for Rafa. So right now I played Rafa. It's not a small deal," the 24-year-old said.

A break in the second set at one stage saw Wu take the lead. But both players had trouble on serve and Nadal went on to win the match in 86 minutes.

It was 29-year-old Nadal's first match since losing in the third round of the US Open said he could play better, and was confident that his form in practice would soon be reflected on the competitive court.

DOUBLE TROUBLE

Meanwhile, Novak Djokovic played doubles with his younger brother Djordje for the first time, notching up a win in the process.

The world no.1 is eight years older than his brother, who is ranked 1502 in the world. A healthy crowd watched the Djokovics win 7-6 (7-5) 6-7 (6-8) 10-5 over Gong Mao-Xin and Michael Venus.

"The crowd got into it. It was exciting. I'm very happy for my brother, that he was able to experience such an atmosphere," Djokovic said.

"He never did that before. He never experienced such a great crowd. He never played in a big stadium. We enjoyed it very much -- encouraging each other to the first win."