The comeback Serena Williams has made to tennis from having a child last year is nothing short of extraordinary. After withdrawing from the Australian Open earlier in 2018, Williams made it to the fourth round of the French Open before withdrawing due to injury; and then she made the final at Wimbledon before losing to Angelique Kerber. However, according to Williams, that comeback hasn't come without some frustrating side effects.

Williams tweeted on Tuesday night that she was "randomly" drug tested again, and bemoaned the frequency with which she's being tested due to "discrimination." She concluded with the silver lining that "at least I'll be keeping the sport clean."

The USADA database was last updated July 20, but it says that Williams has been tested five times in 2018 (which is already known from a Deadspin piece report between the French Open and Wimbledon about a bizarre encounter between Williams and a USADA agent, in which the agent waited for Williams in her home before Williams refused to submit to the test). This would mark the sixth time that Williams has been tested this year.

Other top tennis stars, including Australian Open winner Caroline Wozniacki, French Open winner Simona Halep and Wimbledon winner Kerber haven't been tested this year at all. Others, including No. 3 Sloane Stephens, have been tested just once. Maria Sharapova, who was suspended for doping when she tested positive for meldonium in 2016, doesn't have any tests on record.

Williams has won 23 Grand Slams, and she's chasing No. 24 to tie the record set by Margaret Court. Now 36, Williams came close at Wimbledon, and she'll have another shot at the US Open in August and September.