Serena Williams has begun training again after a series of health problems including blood clots on her lung.The Wimbledon champion used her Twitter feed to announce her return to the court to her two million followers.Linking to a picture of herself serving in a bright pink body suit, she wrote: "Look who I spotted on the court. Her first day back..."Williams' agent Jill Smoller said the 29-year-old American was "taking it day by day depending on how she's feeling".Former world number one Williams has not played competitively since winning Wimbledon-her 13th Grand Slam title - last July.The American has been unable to compete after cutting her foot on glass in a restaurant when celebrating her triumph at the All England Club.She required two operations on her foot and in February suffered a pulmonary embolism that led to an emergency procedure in Los Angeles.Williams said in March that she was targeting a return to competitive action in "early summer".Source: BBC