The guy who was Serena's hitting partner at Wimbledon was more powerful then Serena.
Same goes for Jelena and for Jennifer. They couldn't beat their male hitting partners and these guys don't even have a ranking. They're hired by the WTA simply to provide hitting partners at the large events.
The guys simply hit harder but more importantly they hit with more spin.
It's no secret amongst the pros and especially the people who work with the players that the top female players wouldn't even get a ranking on the ATP.
Sorry, I know this is always been an argument amongst fans and people who work on the fringes of the game (media etc.) but it's not an argument with the players and their coaches.
Official website of Jelena Dokic(CLOSED) We came, we saw, we conquered and we left