Five-time Grand Slam champion Maria Sharapova came back from a set down to defeat American Alison Riske at the Shenzhen Open on Tuesday and book a spot in the quarter-finals.

Riske, who had been hoping for her first win against the former world number one, proved to be a tough opponent early in the match before the Russian finally overcame her 3-6, 6-4, 6-2 in just under two and a half hours.

"I'm glad I got this kind of match. Although I would've wanted a quick and easy victory, these are the kind of matches that you need, especially with such a short preparation going into the Australian Open in terms of matchplay," Sharapova said afterwards.

"I'm just glad that I came back. She played a really great match and as I said to her at the net, I thought it was a great quality match. In the end, I won it so I have another opportunity to keep going."

Despite Riske's early lead, Sharapova hit 34 winners, nearly three times Riske's total.

The world number 59 was helped by 11 aces, and managed to save seven out of 10 break points.

French Open champion Jelena Ostapenko meanwhile slumped to a shock defeat on Tuesday only days after beating Serena Williams in the American's comeback match.

Number two seed Ostapenko of Latvia lost to the big-serving Czech Krystina Pliskova 6-1, 6-4 in just an hour and 20 minutes.

Ostapenko had beaten Williams via a super tie-break 6-2, 3-6, 10-5 in Abu Dhabi on Saturday, but she had no answer to the power of Pliskova, identical twin sister of former world number one Karolina.

"I felt a bit nervous in the second set, but my serve was really working today, which was a big help," Pliskova said after the match.

Pliskova, third on the WTA Tour's ace charts last season, raced out to a 5-0 lead to start the match.

"My serve was great today, especially in the first set. I feel like it's really fast here, so that can be tricky for both of us, and the first of us playing fast won the point."

It was not the start to 2018 that Ostapenko wanted, with the first Grand Slam of the year -- the Australian Open -- starting in 13 days' time.

Earlier there was disappointment for the home crowd when Chinese number three seed Zhang Shuai was bundled out in three sets by Kazakhstan's Zarina Diyas.