Top seed Iga Swiatek reached the third round of the women's singles with a tenacious straight-set win over Petra Martic on Centre Court at Wimbledon.

Croatian Martic pushed the Polish world number one in moments but lost her serve late in both sets in Swiatek's 6-4 6-3 win.

She also needed a lengthy medical timeout for a back injury in the opening set.

Swiatek's victory extended her formidable win streak to 21 matches.

The 23-year-old is a five-time Grand Slam winner, although she has struggled on grass in the past, and has never advanced beyond the quarter-finals at SW19.

Swiatek will face Yulia Putintseva in the third round with the Kazakh player upsetting Czech 27th seed Katerina Siniakova 6-0 4-6 6-2 earlier on day four.

Elsewhere in the women's draw, American fifth seed Jessica Pegula became the highest-ranked casualty at this year's singles when she was beaten by world number 42 Wang Xinyu.

The Chinese player won 6-4 6-7 (7-9) 6-1 on court three to set up a third-round tie with Britain's Harriet Dart.

Pegula had looked in formidable form on grass, reaching the final of Eastbourne and winning her first-round tie at Wimbledon in just 49 minutes.

But plenty of other seeds recorded dominant straight-set wins on a wind-affected day in south London.

Ons Jabeur, runner-up at the past two Wimbledons, beat American Robin Montgomery 6-1 7-5.

The Tunisian 10th seed will take on 21st seed Elina Svitolina in the third round after the Ukrainian overcame German Jule Niemeier 6-3 6-4.

American 11th seed Danielle Collins beat Hungary's Dalma Galfi 6-3 6-4 and will face 20th seed Beatriz Haddad Maia next after the Brazilian benefitted from a walkover.

Madison Keys, a quarter-finalist last year's, beat China's Wang Yafan 6-2 6-2, while 2017 French Open champion Jelena Ostapenko defeated Ukraine's Daria Snigur 6-3 6-0.