Ukrainian wildcard Elina Svitolina beat Belgian 28th seed Elise Mertens to reach the third round at Wimbledon on day four.

Svitolina dominated the opener before appearing to struggle with something in her eye in the second, but she regained form to take victory.

On court 12, French Open runner-up Karolina Muchova suffered a shock first-round exit to Jule Niemeier.

The Czech 16th seed sustained a leg injury after falling over in the deciding set and went down 6-4 5-7 6-1 to German Niemeier, who reached the quarter-finals at Wimbledon last year but is ranked 87 places below Muchova.

Svitolina's win against Niemeier might be seen as an upset, but the world number 76 has reached the semi-finals of two Grand Slams and only fell down the rankings after stepping away from the sport to have a baby last year.

She returned to the tour in April following the birth of her daughter in October 2022, also reaching the French Open quarter-finals last month and securing a comprehensive first-round Wimbledon victory over seven-time Grand Slam champion Venus Williams.

Svitolina will face 2020 Australian Open winner Sofia Kenin in the third round after the American qualifier eased past China's Wang Xinyu 6-4 6-3 on Thursday.

Two other Ukrainians, Lesia Tsurenko and Anhelina Kalinina, also secured wins, with Kalinina waiting an extra two days to play her first-round match against Spain's Jessica Bouzas Maneiro - which she won 6-4 6-3 - because of rain delays at the All England Club.

Tsurenko moved into the third round for the third time with a straightforward 6-4 6-1 win over Czech player Katerina Siniakova.