At 24-24 Anderson took a tumble but managed to get to his feet and hit a left-handed shot inside the baseline, taking the point for 0-30.
Spare a thought for Rafael Nadal and Novak Djokovic, whose wait to play their own semifinal finally ended around 8 p.m. local time. He would go on to break the big American and then served out the match.
Isner called for a trainer to have his right index finger treated at 2-1 but didn't seem bothered by any problem when play resumed.
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In a match full of booming serves and countless aces, the first three sets were all decided on tie-breaks. The last point of the set was briefly held up as a ball boy had to be substituted after hurting his shoulder.
Anderson was coming off a huge upset win in the quarterfinals over Roger Federer. The only Wimbledon finalist from South Africa was Brian Norton in 1921 while John is trying to become the 12th American Wimbledon finalist in the Open era and the first overall in Majors since Andy Roddick in 2009.
The three-time Wimbledon champion posted a video to his Instagram story of him playing marbles in the dressing room as he prepares to face Nadal.
Isner was trying to end the longest major title drought - 58 majors - for US men in tennis history. Isner has won all 95 of his service games so far.