US Open Day 8Updated 2158 GMT (0558 HKT) September 5, 2016 Chat with us in Facebook Messenger. Find out what's happening in the world as it unfolds. Photos: US Open second week in full swingJuan Martin del Potro, pictured, faced Dominic Thiem in the fourth round of the US Open on Monday. Hide Caption 1 of 8 Photos: US Open second week in full swingAnd when the injury hit del Potro called for the trainer after three games, things looked bleak for the 2009 winner. Hide Caption 2 of 8 Photos: US Open second week in full swingBut del Potro appeared unaffected while French Open semifinalist Thiem subsequently injured his knee. Hide Caption 3 of 8 Photos: US Open second week in full swingThiem had to retire trailing 6-3 3-2 and del Potro was into his first grand slam quarterfinal since 2013. Hide Caption 4 of 8 Photos: US Open second week in full swingHe next faces Stan Wawrinka. The twice grand slam winner -- who saved a match point in the third round -- defeated Illya Marchenko 6-4 6-1 6-7 (5-7) 6-3.Hide Caption 5 of 8 Photos: US Open second week in full swingElsewhere, Karolina Pliskova saved a match point to beat Venus Williams 4-6 6-4 7-6 (7-3) to make a first major quarterfinal. Hide Caption 6 of 8 Photos: US Open second week in full swingWilliams, who won the last of her seven majors in 2008, held the match point at 5-4 in the third on the Pliskova serve. Pliskova saved it with a gutsy swinging volley. Hide Caption 7 of 8 Photos: US Open second week in full swingBut there were no issues for her younger sister, Serena. The top seed crushed Yaroslava Shvedova 6-2 6-3 to record a leading 308th grand slam win. Hide Caption 8 of 8Juan Martin del Potro reached the quarterfinals of a grand slam for the first time in three years, while Serena Williams and Venus Williams also saw action in the day session. More from Sport - TennisSanchez-Casal Academy: The making of Andy MurrayMaria Sharapova: Russian wins on return from 15-month doping banMaria Sharapova's comeback leaves women's Tour dividedSerena Williams slams Ilie Nastase over 'racist' commentsCan Serena Williams return to the top of tennis after giving birth?