Novak Djokovic of Serbia reacts during a second round men's singles match against Ryan Harrison of the United States at the All England Lawn Tennis Championships at Wimbledon, England. (AP)

Novak Djokovic of Serbia returns a shot against Ryan Harrison of the United States at the All England Lawn Tennis Championships at Wimbledon, England. (AP)

Novak Djokovic of Serbia returning a shot against Ryan Harrison of the United States at the All England Lawn Tennis Championships at Wimbledon, England. (AP)

Novak Djokovic of Serbia returning a shot against Ryan Harrison of the United States at the All England Lawn Tennis Championships at Wimbledon, England. (AP)

Another shot of Novak Djokovic returning a shot against Ryan Harrison at the All England Lawn Tennis Championships at Wimbledon, England. (AP)

Ryan Harrison of the United States plays a return to Novak Djokovic of Serbia during a second round men's singles match at the All England Lawn Tennis Championships at Wimbledon, England. (AP)

Ryan Harrison of the United States returns a shot to Novak Djokovic of Serbia at Wimbledon, England. (AP)

Another shot of Ryan Harrison of the United States returning a shot to Novak Djokovic of Serbia at Wimbledon, England. (AP)