Players that were major chokers but improved mentally.
I'm not just talking about in slams (like Clijsters) but all the time. The ATP have Berdych & Soderling - two players who got to the top 20 on mostly talent alone and were weak mentally but improved that area and moved up to top 10 level.
I think Petkovic had quite a choking thing going on for a while. But she was putting that to rest quite well before she got injured.
Re: Players that were major chokers but improved mentally.
Henin's probably the most famous example. That choke against Clijsters at the 01 French Open was epic.
Serena was a mental disaster for most of 2001. She was up 5-3 on Hingis in Australia and lost, she served for the match against Capriati at Wimbledon and lost, she blew like six match points against Seles in one of the US Open tuneups, and it took her forever to close out Davenport of all people at the US Open.
And then there's Clijsters. I remember she had quite an epic choke against Henin in Berlin in like 03 or something. But she always played scared against Henin in her first career.