Zhang Shuai (file photo)

PARIS, May 29 (Xinhua) -- China's top-ranked Zhang Shuai made comebacks in both sets to win 7-5, 6-4 against Donna Vekic to make into the second round, while her compatriot Zheng Saisai failed to shock the world No. 3 Karolina Pliskova due to her knee injury, losing 7-5, 6-2 here on Monday.

Men's singles reigning champion Novak Djokovic and his counterpart in women's singles Garbine Muguruza both recorded comfortable victories to kick off their title defence.

The tournament's No. 32 seed Zhang held her serves twice with Vekic to tie the game at 2-all, before Vekic had the first break in the following game, and led 5-2 into the eighth game with some tricky hits.

Zhang then took five games in a row to turn around at 7-5.

In the second set, Vekic came back from 2-1, 40-0 down in the fourth game to ride on a three-game winning streak for a 4-2 advantage.

Again, it was Zhang who kicked off another surge for four games in a row, highlighted by an ace in the ninth game, and she sealed the hard-fought victory on her opponent's double fault.

"I had to admit that I didn't play very well today despite the win. Actually, I have shown some qualities although I lost time and time again in the past several weeks," said Zhang, who set up a second-round clash with Aliaksandra Sasnovich of Belarus, whom she beat in Australian Open early this year.

With bandage on her right knee, Zheng once built a 5-4 lead in the opening set, before only nailing two more games with bad movement on clay.

Zheng was overwhelmed with pity that she could not put 100 percent effort into the match due to a knee injury.

Zheng recorded 45 percent in her win from the first serve, 24 percent lower than her opponent.

"It was awful that I failed to spare no effort in the game (due to a knee injury)," she said.

Djokovic, bidding to become the first man to have won each of the four Grand Slams at least twice in the Open Era, swept Marcel Granollers of Spain 6-3, 6-4, 6-2 in two hours and 27 minutes.

It was the first match after the Serbian announced his partnership with Andre Agassi, an eight-time Grand Slam singles winner.

"I wanted to start off well, engage all the engines in the right way and come out with the right intensity," said Djokovic.

Muguruza powered past Italian veteran Francesca Schiavone, the 2010 champion and 2011 runner-up, 6-2, 6-4.

The Spanish star paid her tribute to the 36-year-old Schiavone by calling the latter a "legend" after the comfortable win.

Also on Monday, Rafael Nadal used a 6-1, 6-4, 6-1 victory over Benoit Paire as his first step to lift his 10th trophy at Roland Garros. Seeded players, including Milos Raonic, Marin Cilic and David Goffin, met little resistance in their quest for a second-round spot, while the 30th-seeded David Ferrer was involved in a four-and-half-hour marathon match to beat Donald Young in full sets.

The remaining first-round matches of the singles will be held on Tuesday, with men's top seed Andy Murray and third seed Stan Wawrinka set to step on the court. China's Peng Shuai and Duan Yingying will also feature in women's matches.