Conversation so far:
When did Pancho Gonzales play open tennis?
Most of Gonzales's career as a professional took place before the start of the open era of tennis in April 1968.
What happened in that year?
he was therefore ineligible to compete at the Grand Slam events between the end of 1949 (when he turned pro) and the start of the open era in April 1968.
why was he ineligible to compete?
CANNOTANSWER.
What happened after that?
In May 1968, he was the first professional to lose to an amateur.

Question: who did he lose to?
Self-contained rewrite: who did Gonzales lose to