Summary:
A tennis player serves a 100mph serve into the head of her opponent's opponent as they lost in the fifth round of the US Open.

Full article:
By . Steven Donaldson . It's a shot normally seen on park tennis courts on a Saturday morning, not the lush green grass of Wimbledon. But Klara Koukalova showed that even professional players get it wrong sometimes when she served the ball into the back of her doubles partner's HEAD. Monica Niculescu was crouching at the net but felt the full force of the 100mph serve. She was consoled by Koukalova and had to sit at the side of the court to recover. The pair were taking on 16th seeds Garbine Muguruza and Carla Suarez Navarro but lost after three sets, 6-2, 4-6, 6-4. VIDEO Scroll down to watch the day five update from Sportsmail's Whitwell and Fraser . Ready: Klara Koukalova throws the ball up to serve . Set: Koukalova prepares to serve the ball to her opponents . Go: Koukalova sends the ball straight into Monica Niculescu's head at 100mph . Oops: Niculescureels after being hit in the head as her partner realises what she has done . Ouch: Koukalova nurses her injury after being struck on the back of the head . Comforted: Koukalova makes sure Niculescu is OK after the incident . Resting: Koukalova takes a seat while Niculescu recovers . VIDEO Sock hits lineswoman in the face .