LONDON (AFP) - Women's tennis world No. 1 Ashleigh Barty and defending champion Angelique Kerber eased into the second round of Wimbledon on Tuesday (July 2).

Unlike second seed Naomi Osaka on Monday, who exited in straight sets, both Barty and Kerber disposed of their opponents with relative ease.

Barty beat China's Zheng Saisai 6-4, 6-2 while Kerber, the fourth seed, beat fellow German Tatjana Maria 6-4, 6-3.

Barty did not have it all her own way in the first set being pegged back to 4-4 after leading 3-0.

Barty finally took the set in breaking Zheng on her serve at 5-4.

The Australian made the break on Zheng's first service game in the second set and was rarely troubled after that.

"The first round is always very tough and it took a little time to get used to the court," said Barty. "It definitely took some time to adjust."

Barty, who is bidding to become the first woman since Serena Williams in 2015 to win both the French Open and Wimbledon in the same season, said the weight of being No. 1 did not affect her particularly.

"It is a bizarre feeling to be honest but I try and go about my business as usual," she said.

Barty plays tricky Belgian Alison van Uytvanck, who knocked out then defending champion Garbine Muguruza last year in the second round, for a place in the last 32.

"It will be an exceptional challenge and it will be important for me to be strong on my serve," she said.

Kerber, who came into the tournament on the back of being the beaten finalist at Eastbourne last Saturday, looked to be cruising to victory when she broke Maria in the first game of the second set.

However, her compatriot fought back to break her in the next game after a long tussle which saw Kerber get increasingly annoyed by her errors.

Kerber, though, again broke in the following game a sweeping forehand cross-court provoking her to a double fist shake and a yell of delight.

There were further breaks of serve but in the end Kerber came through safely.

In men's play, Japan's Kei Nishikori eased into the second round, beating Brazilian qualifier Thiago Monteiro 6-4, 7-6 (7-3), 6-4.

The 29-year-old eighth seed, a quarter-finalist at Wimbledon last year, never gave Monteiro a hope of an upset.

Nishikori - whose best Grand Slam performance was reaching the 2014 US Open final - will play either experienced Uzbek Denis Istomin or Britain's Cameron Norrie in the second round.

But two-time French Open finalist Dominic Thiem was knocked out, losing to veteran Sam Querrey 6-7 (4-7), 7-6 (7-1), 6-3, 6-0.

The 25-year-old Austrian fifth seed does not find grass to his liking - his best showing reaching the last 16 in 2017 - and wilted under the expert serve-and-volley game of the American.

Querrey, a semi-finalist at Wimbledon in 2017, will next play Andrey Rublev of Russia or Cristian Garin of Chile.

Thiem joins fellow top-10 seeds Stefanos Tsitsipas and Alexander Zverev in being toppled in the first round.

The big-name men will come to the fore later on Tuesday with eight-time champion Roger Federer playing Wimbledon debutant Lloyd Harris of South Africa.

His fan club were there on Centre Court in force with red sweaters emblazoned with "Roger the greatest of all time".

Federer's great rival Rafael Nadal will also play his first match of this year's championships, the two-time champion facing Japanese qualifier Yuichi Sugita.

If Nadal comes through, he will renew his bitter rivalry with Nick Kyrgios after the combustible Australian claimed a 7-6 (7-4), 3-6, 7-6 (12-10), 0-6, 6-1 win over countryman Jordan Thompson.

Kyrgios, who stunned Nadal as a 144th-ranked wild card on his tournament debut in 2014, put in a typically raucous and charismatic display on Court 3, where he fired 23 aces and 63 winners.

Meanwhile, Kate, the Duchess of Cambridge, left the plush confines of the Royal Box at Wimbledon to watch a British wild card competing on the lowly outside courts.

In a rare move for a member of the royal family visiting the All England Club, Kate took her place among the 318 regular seats on Court 14 to watch Harriet Dart take on American Christina McHale.