After playing very high level tennis in 2002-2003 against 8 quality players (Venus, Davenport, Justine, Capriati, Hingis, Seles, Kim, Amelie), she burned out and played part-time since, together with Venus.
It took 5 years for her to be competitive again, but the field had changed. 2009-2011 were the time of slamless #1. Now the top 10 is: Masha, Vika, Aga, Errani, Li, Kerber, Kvitova, Caro, Makiri. It's a bit stronger with Masha, Vika, Li, and Kvitova. But they are no Nadal, Nole, Murray, Del Potro. More like Safin, Roddick, Ferrero, Hewitt to Federer.
Sorry, no comparison with Federer. Those who dare to say Serena was better in terms of H2H with her chief rivals, should have their heads examined. Federer has FAR greater competitors and he's still far from game over.